We describe the seasonal abundance of caterpillars in the central Brazilian cerrado (savanna woodland) from 1991 to 1995 and discuss possible explanations for abundance variation. The climate is highly seasonal with a marked wet season between October and March. On 15% of a total of 10 800 censuses of host plants (nine species, three genera and three families) we found caterpillars feeding externally. We successfully reared 247 lepidopteran species (32 families), 61 % of which occurred at low density (fewer than four times). The frequency of plants with caterpillars increased sharply at the beginning of the dry season, in mid-April, and remained high until mid-July and then declined sharply, reaching its lowest point in the early-wet season (October). Some caterpillar species are present on food plants throughout the year, whereas others are highly seasonal, occurring only in the wet season. The duration of the pupal stage and its variance increased at the end of the wet season. Possible explanations for the low caterpillar density at the beginning of the rainy season include accumulated water stress in the late-dry season, leaf defenses, delay of adult emergence, and predator and parasitoid activities. The higher caterpillar abundance during the first half of the dry season may correspond to the availability of temporary enemy-free space.