By a previously established test procedure that uses 4.5 micromoles phosphate in the medium, the marine red alga Champia parvula (C. Agardh) Harvey was shown to be more sensitive to arsenite (As(III)) than to arsenate (As(V)). Mature cystocarps (evidence of sexual reproduction) were formed when plants were exposed to 65 micrograms As(III)/L, but not when exposed to 95 micrograms As(III)/L. Three hundred micrograms As(III)/L was sufficient to kill C. parvula. Using the standardized test, plants grew just as well in 10,000 micrograms As(V)/L as they did in the controls; however, sexual reproduction did not occur at this concentration. Phosphate concentration up to 9.1 micromoles had little or no effect on As(III) toxicity. Arsenate toxicity, however, was inversely proportional to the amount of phosphate in the medium. In the absence of added phosphate, As(V) toxicity was similiar to that of As(III), except plants were still alive at 1,076 micrograms As(V)/L (although they did not grow).