Gastrointestinal and serum absorption of astaxanthin was studied in rainbow trout, Oncorhynchus mykiss (Walbaum) (217 + 2 g) fed diets supplemented with either esterified astaxanthin (from Haematococcus pluvialis) or free astaxanthin (synthetic, as 8% w/w beadlets) at similar levels (50 mg kg À1 ). After 56 days of feeding, there was a significant difference (P 0.0582) between steady-state serum astaxanthin concentrations for fish fed free (2.0 + 0.3 mg mL À1 ) or esterified astaxanthin (1.3 + 0.1 mg mL À1 ) at the 90% confidence level. However, following ingestion of a single meal supplemented with free or esterified astaxanthin, the rates of astaxanthin absorption into serum were not significantly different (P . 0.1) (0.8 + 0.2 mg mL À1 h À1 and 1.0 + 0.4 mg mL À1 h À1 respectively). In fish fed both free or esterified astaxanthin, higher absorption (P , 0.05) of astaxanthin by the ileal (0.8 + 0.14 mg g À1 and 0.9 + 0.15 mg g À1 respectively) compared with the posterior (0.2 + 0.01 mg g À1 and 0.3 + 0.14 mg g À1 respectively) intestine was recorded. This confirmed the role of the anterior intestine in carotenoid absorption. Non-detectable levels of esters in digesta taken from the hind intestine suggest the anterior intestine is also the primary region for ester hydrolysis.