The occurrence of growth hormone (GH) binding was investigated in the gilthead sea bream larva, Sparus aurata, using recombinant trout GHII (rtGH) as a tracer. GH binding was detected in all the specimens examined (5, 17, 27 and 40 days after hatching). At 40 days after hatching, we observed a concentration of GH binding in the head region. Specific binding was dependent on protein membrane concentration and Scatchard plots showed a single class of high affinity and low capacity binding sites. Specific binding was inhibited by rtGH and ovine GH in a dose dependent manner. Chinook salmon prolactin and chinook salmon gonadotropin did not compete for 125I-rtGH binding sites. To our knowledge, this is the first report that indicates the presence of specific and saturable GH binding sites during early growth and development of a fish species.