Phytoplankton species were grown in batch cultures in the presence of the water-soluble fraction (WSF; 50 and 100%) of a Mexican crude oil (Isthmus Cactus). The algae exhibited various responses ranging from retarded growth to stimulation of growth. The cellular content of chlorophylla and the lipid composition of the algae were examined. Four algae, the bacillariophytesNitzschia closterium andAsterionella glacialis, the cryptophytesRhodomonas lens, and the chlorophyteDunaliella tertiolecta, exhibited retarded growth. In most of these algae, cellular chlorophylla, lipid pigments, glycolipids and triglycerides decreased whereas sterols and hydrocarbons accumulated. Phospholipids did not exhibit any specific pattern of change during the experiments. The cyanophyteAgmenellum quadruplicatum and the bacillariophyteSkeletonema costatum were less sensitive to the WSF. The cell yield of the dinophyteProrocentrum minimum was stimulated by the WSF. In these three latter species, lipid pigments were enhanced or remained at control levels. We concluded that the toxic effect of the WSF disrupts the biosynthesis mechanisms required for a functional photosynthetic apparatus (biosynthesis of chlorophylla, glycolipids and lipid pigments) in sensitive algae, a phenomenon coupled to sterol accumulation in these algae.