1. A fungus, Acrostalagmus aphidum Oud., parasitizes aphids in Porto Rico. It had been reported as A. albus Preuss. 2. The size of the spores in the various natural strata is more or less uniform. 3. The size of the spores appears to be larger for those produced on the natural strata than those developing on oatmeal agar cultures. 4. There seems to be a wider range of length and width of spores on our form than on the A. aphidum Oud. described in Saccardo's "Sylloge Fungorum". 5. Some of the conidia become one-to several-septate prior to germination. 6. Conidia or head-like structures are produced on germinating spores. 7. Some young conidia germinate while still attached to the head or to the branch tips. 8. Spores germinate rapidly in sugar solutions. 9. Spores rapidly lose their germinating power when dried. 10. Experiments indicate a possible stimulating effect of aphid juices on germination and development of spores. There are probably variations in the degree of that influence, according to species. 11. The fungus grows best at reactions of pH 6.93 to 7.93. 12. So far as is known, Acrostalagmus aphidum attacks the aphids on 17 species of the higher plants. Of these 8 had been reported previously and the remaining 9 are new additions. Among these are important crop plants. 13. The aphids which may be parasitized comprise five different genera of which two are here first reported. Aphis pseudobrassicae is first here reported parasitized by the fungus. 14. The fungus does not parasitize the corn aphid under natural conditions. 15. The fungus has also been reported on the eggplant lace-bug, Corythaica monacha. 16. Acrostalagmus aphidum can be employed successfully and cheaply in the control of the aphids of the eggplant. The method will probably be effective in field control of the aphids of other vegetables. 17. The pathogene lives in greenhouse soil. No experiment proof is at hand which would demonstrate whether the fungus is eliminated by the application of fungicides used for the control of plant diseases.Resumen en español