SYNOPSIS.The nutritional requirements of 3 isolates ofPrymnesium parvum(2 Israeli, 1 Scottish) included vitamin B12and thiamine. For comparison, 2 other brackish chrysomonads were studied:Monochrysis lutheriisolated by Droop in Scotland andIsochrysis galbanapurified by McLaughlin from a culture obtained from the Plymouth laboratory.The isolates ofPrymnesium parvumandIsochrysis galbanahad a molecular B12specificity likeOchromonas malhamensis: no response to Factor B, pseudovitamin B12, Factor A or Factor H.M. lutheri, in contrast, responded to pseudovitamin B12, Factor H, and Factor A.Thiamine was essential; 1.0 μg.% allowed full growth ofP. parvum.The NaCl concentration for good growth was 0.3–5.0%; growth was possible from 6–12%. Dark growth was not achieved.Ammonia, as suggested from its use in suppressing outbreaks ofP. parvum, was sharply inhibitory, less so at high concentrations of NaCl or at acid pH.Nitrate, ammonia, arginine, asparagine, methionine, histidine, alanine, glycine, serine, proline, leucine, isoleucine, tyrosine, aspartic and glutamic acids, acetylurea, and creatine served as nitrogen sources in both acid and alkaline media.The phosphate requirement ofP. parvumandM. lutheriandIsochrysis galbanawas satisfied by inorganic phosphate, commercial glycerophosphate, yeast adenylic acid, cytidylic acid, monoethyl phosphate, and riboflavin monophosphate.Laboratory cultures in defined media of the isolates ofP. parvumwere toxigenic toLebistesandGambusia.Culture fluids from alkaline media were more toxic than those from acid media, as previously noted in Israel.Culture media suitable for production of large quantities of these organisms were devel