ABSTRACTEvidence of several kinds was sought to confirm that structures thought to be resistant sporangia (RS) inNeocallimastixsp. shared characteristics with RS of other zoosporic fungi. In liquid medium,Neocallimastixformed a chytrid-like thallus made up of a system of intramatrical anchoring rhizoids and an extramatrical sessile or stalked zoosporangium on fiber pieces. As the culture aged, there was a shift from the development of zoosporangia to the production of RS. RS were first observed 36 h after zoospores were inoculated into fresh basal anaerobic medium. Encysted zoospores often developed into elongate thalli that branched monopodially near the tip. During RS formation, the tip of each branch enlarged and the nuclei and cytoplasm migrated towards this enlarged region. A septum was then formed to delimit the developing RS which became pigmented as it matured. The mature RS were dark brown and the pigment in their walls was identified as melanin using histochemical tests. Zoospores had the 1C level of DNA with 1C and 2C levels being observed in developing zoosporangia. The DNA content in the mature RS (4C) was found to be twice the highest level of DNA in the zoosporangium (2C) which suggested that there was a diploidization during RS formation. Additional studies needed to characterize the RS are discussed.