A statistical evaluation was made of the daily rainfall at Caguas and Fajardo, covering 70- and 60-year periods. The purposes of this evaluation were: 1, To investigate the character, if any, of the rainfall distribution and of its deficiency (drought) characteristics; and 2, in the event that any character was found in rainfall in the sense of distribution and deficiency, to develop a Rainfall Deficiency Index which would be useful to agricultural and other programs and inventories. The study first consisted of determining the least rainfall in 60, 120, and 180 consecutive days, and the maximum rainfall in 30 and 90-day periods of each year. This analysis suggested the occurrence of most deficient and least deficient rainfall in 120 (or 180) day cycles during the drier part of the year within 10- to 11-year periods; the Caguas data beginning in 1899 and in 1909 at Fajardo. The suggested cycles appear to be more defined at Caguas than Fajardo. It appears that years ending in 6 or 7 almost invariably seemed to be among the driest in their respective decades; 1899 to 1968. Years ending in 1 or 2 seemed identified with least deficient and better distributed rainfall. This preliminary analysis was based mainly on the least rainfall in 120 days parameter. It was found that the ratio of the mean least rainfall in 120 days of the dry years (ending in 6 or 7) to the "wet" years (ending in 1 or 2) is 0.364 for Caguas and 0.603 for Fajardo. Also, the coefficient of variation of the data for dry and "wet" years was lower than when calculated for all years. Contrast between dry and wet years is apparent even for the least rainfall in 180 days (6 months of the year). In this respect the ratios of dry to "wet" years are 0.494 and 0.595 for Caguas and Fajardo, respectively. There is no apparent contrast, however, when the annual, or even maximum, rainfall in 90 days at the two locations is compared for "wet" and dry years. A Rainfall Deficiency Index (DEF) is proposed. Such deficiency is normally distributed for Caguas and Fajardo. This index integrates the deficiency and distribution characteristics of the annual rainfall. It can be used, therefore, as a guide for evaluating the rainfall characteristics in the agricultural regions of Puerto Rico and for the determination of priorities for supplemental irrigation. A Rainfall Distribution Coefficient (DIS) was developed as a direct means of describing the distribution characteristics of annual rainfall, and it accounts indirectly for the rainfall deficiency within a given area or areas of similar annual rainfall. DEF accounted for 74.0 and 64.4 percent of the variations in DIS at both Caguas and Fajardo.Se evaluaron estadísticamente los datos diarios sobre la lluvia en Caguas y Fajardo durante 70 y 60 años, respectivamente. Los objetivos principales de la evaluación fueron: 1, Investigar el patrón de distribución anual de la lluvia, si es que existe y 2, de ser así, desarrollar entonces un índice de Deficiencia de Lluvia (DEF), que pueda ser útil para la planificación agrícola de Puerto Rico. El primer paso fue determinar para ambas áreas (Caguas y Fajardo) la cantidad mínima de lluvia durante períodos de 60,120 y 180 días consecutivos, así como la máxima durante 30 y 90 días, en cada uno de los años comprendidos en el estudio. Este análisis de por sí sugiere que durante la temporada de sequía tiene lugar una incidencia cíclica de lluvia particularmente deficiente, que coincide con los años que terminan en 6 ó 7. Así mismo se observó que en ciclos de 10 a 11 años la lluvia es generalmente menos deficiente durante la temporada de sequía en aquellos años que terminan en 1 ó 2. En relación con este hecho, el coeficiente de lluvia mínima en 120 días de los años más secos (los que terminan en 6 ó 7) respecto al de los menos secos (los que terminan en 1 ó 2) es 0.364 para Caguas y 0.603 para Fajardo. El contraste, por tanto, es más evidente en el caso de Caguas. Los coeficientes de variabilidad de los respectivos datos, tanto para los años secos como para los menos secos, resultan más bajos que cuando el cálculo se hace para todos los años. Para caracterizar la distribución y la deficiencia de la lluvia anualmente se propone un índice de Deficiencia (DEF), que permite hacer una determinación numérica. Este índice puede ser útil para fines de comparaciones hídricas entre áreas y establecer así prioridades respecto al suministro de riego suplementario.