SUMMARYInvestigations on an isolate ofBipolarissp. demonstrate that this fungus displays autoinhibition of conidium germination. Filtrates from 3–7-da-old agitated liquid cultures permitted germination in less than 5% of the conidia tested. Addition of nutrients to 5-da filtrates failed to overcome inhibition completely. The inhibitory activity of filtrates was unchanged by freezing, autoclaving, or concentration. An inverted-culture bioassay showed that the time course for the production of inhibitory factors in agar cultures was similar to that seen in liquid cultures, and that comparable patterns were observed when different carbon and nitrogen sources were tested. The germination of conidia of six Deuteromycetes and the sporangiospores of one zygomycete was delayed or inhibited when incubated on inverted-agar cultures ofBipolaris. Bipolarisconidia incubated on the upper surface of an actively growing colony yielded 3–5% germination at the center of the colony and 95–99% germination over the colony margin and uncolonized medium. Conidia incubated at densities greater than 10×102spores/ml exhibited decreased germination percentages compared to more dilute suspensions. Growth of germ tubes was not as susceptible to the action of inhibitory filtrates as was conidium germination, and it was shown that conidia which had passed the latent period in germination were no longer susceptible to the inhibitor.