Zebrafish is an often used model of vertebrate lipid metabolism. In this article, we examined the effects of diets rich in fish oil, a dietary fat that has been shown to have antiobesity effects in mammals, or lard on body fat accumulation in zebrafish. Adult female zebrafish were fed a high-fat diet containing 20% (w/w) fish oil or lard for 4 weeks. Fish in the fish oil diet group had less body fat accumulation compared with those in the lard diet group. In the intestine, expression of genes for the alpha (hadhaa) and beta (hadhb) subunits of the beta-oxidation enzyme hydroxyacyl-Coenzyme A dehydrogenase/3-ketoacyl-Coenzyme A thiolase/enoyl-Coenzyme A hydratase was significantly increased in the fish oil diet group compared with the lard diet group (p < 0.05). In the liver, expression of the gene for fatty acid synthase (fasn) was significantly decreased in the fish oil diet group compared with the lard diet group (p < 0.05). These results suggest that the mechanisms underlying the antiobesity effect of fish oil are similar in zebrafish and mammals.