Although thraustochytrid protists are known to be of widespread occurrence in the sea, their hydrocarbon-degrading abilities have never been investigated. We isolated thraustochytrids from coastal waters and sediments of Goa coast by enriching MPN isolation tubes with crude oil. Three isolates tested showed positive hydrophobicity of cell walls as judged by the Microbial Adhesion to Hydrocarbons (MATH) assay. Addition of Bombay High crude oil to nutrient broth slightly enhanced growth of the protists as compared to unenriched controls. Autoclaved crude oil added to sediments was degraded by 2 thraustochytrids to a much greater extent than non-autoclaved oil. Tarballs supported excellent growth of thraustochytrids when added to a peptone broth. Inoculation of thraustochytrids to tarball-enriched sediment resulted in a decrease of up to 71% of tarball contents after a month. Up to 30% of tarballs added to peptone broth was degraded by thraustochytrids in 7 days, as estimated by gravimetry and gas chromatography. Fractions above the retention time for 20 degrees C aliphatics were degraded to a much greater degree than those below 20 C. Thraustochytrids appear to play a definite role in tar ball degradation in sediments.