Isolate 458 of Entomophthora egressa from larvae of the eastern hemlock looper and isolate 521 from larvae of the spruce budworm differed in colony morphology on coagulated egg yolk medium, protoplast growth rate in modified Grace's medium, regeneration sequence, effect on growth medium [pH, osmolality, individual and total ninhydrin-positive compound (NPC) levels, total protein and glucose concentration], rate of glucose uptake, protein synthesis, degree of amino acid utilization and growth response to CO2. The major NPC utilized included t.-aspartic acid, iL-glutamic acid, L-lysine, L-histidine, L-tyrosine, L-leucine, L-valine, I-glutamine, glycine, DL-serine and /3-alanine. The major endogenous protoplast NPC were L-glutamic acid, L.histidine, L-alanine, DLt.-serine and glycine with moderate levels of L.-aspartic acid, t-arginine, i.-proline, t.-glutamine, tL-asparagine and /f-alanine. As a result of the observed developmental and physiological differences, we conclude that isolates 458 and 521 are representatives of two distinct physiological races of E. egressa.