The water mold Blastocladiella emersonii releases zoospore maintenance factor into the medium during zoosporogenesis. Extracellular factor mediates a reversible developmental block that maintains the motile, cell wall-less zoospore phenotype. A method for purifying the factor is reported that results in 75-120% recovery of biological activity. Analyses of purified factor by thin layer chromatography support the conclusion that factor activity resides in a single organic, low molecular weight molecular species. Other data (Gottschalk, W.K. & Sonneborn, D. R. (1985) J. Biol. Chem. 260, 6592-6599) independently support this conclusion and, in addition, support the conclusion that biological activity resides in an SH-containing cyclic ribotide.