Analysis of a natural control agent for the red tide organism (G. breve, P. brevis) indicated the presence of sterols as an active, or adventitious component of a cytolytic mixture elaborated by the cyanobacteriumGomphosphaeria aponinaKützing. Comparisons of several plant sterols showed cholesta‐5,7‐dien‐3ß‐ol (7‐dehydrocholesterol) was cytolytically active towardG. breve. Derivatives of cholesta‐5,7‐dien‐3ß‐ol and cholest‐5‐en‐3ß‐ol (7‐dehydrocholesterol and cholesterol) were prepared to enhance water solubility. Derivatives of the type ROCO(CH2)4CO(OCH2CH2)nOH varied markedly in cytolytic activity withG. breveas the bioassay organism. The derivative of cholest‐5en‐3ß‐ol (cholesterol) (n = 12.5) showed no discernable cytolytic activity up to 20 ppm, but the cholesta‐5,7‐dien‐3ß‐ol (7‐dehydrocholesterol) derivatives (n = 22, 12.5, or 4) were cytolytic at about 5 ppm and had a dose‐dependent response. Implications of the observations are considered.