Several pieces of evidence indicate that Microcoleus chthonoplastes and Phormidium corium, the predominant cyanobacteria in microbial mats on crude oil polluting the Arabian Gulf coasts, contribute to oil degradation by consuming individual n-alkanes. Both cyanobacteria grew phototrophically better in the presence of crude oil or individual n-alkanes than in their absence, indicating that hydrocarbons may have been utilized. This result was true when growth was measured in terms of dry biomass, as well as in terms of the content of biliprotein, the accessory pigment characteristic of cyanobacteria. The phototrophic biomass production by P. corium was directly proportional to the concentration of n-nonadecane (C-19) in the medium. The chlorophyll to carotene ratio of hydrocarbon-grown cyanobacteria did not decrease compared to the ratio in the absence of hydrocarbons, indicating that on hydrocarbons the organisms were not stressed. Comparing the fatty acid patterns of total lipids from hydrocarbon-grown cyanobacteria to those of the same organisms grown without hydrocarbons confirms that n-alkanes were taken up and oxidized to fatty acids by both cyanobacteria.