Earlier work in this laboratory showed that the biological activity of vitamin A extracts from some marine Crustacea was lower than that indicated by the Carr-Price test. It was suggested that this discrepancy might have been due to the presence of cis-isomers of vitamin A, which give the same quantitative colour reaction in the Carr-Price test but are biologically less active than all-trans vitamin A. In this paper, the isomer composition of the vitamin A of two euphausiid Crustacea, Meganyctiphanes norvegica (M. Sars) and Thysanoessa raschii (M. Sars), and of three decapod Crustacea, Pandalus bonnieri Caullery, P. borealis Kroyer and P. montagui Leach, is reported. The percentages of 'fast' (all-trans and 9-cis vitamin A) and 'slow' (13-cis, 9:13-di-cis, neo-b and neo-c vitamin A) reacting isomers were calculated from the rate of combination of vitamin A preparations from these species with maleic anhydride in benzene solution. Hindered cis-isomers were detected by their rapid isomerization with iodine, which results in an increase of the wavelength of the ultra-violet absorption maximum and a large increase in extinction, and by a high value for the ratio (maximum extinction in the Carr-Price test)/ (maximum extinction in the ultra-violet). The retinenes of these vitamin A isomers were allowed to react with opsin in the dark and the neo-b retinene present was measured by the formation of rhodopsin. In all five species studied, the hindered cis-isomer, neo-b vitamin A, was the most abundant form and constituted 53 to 90% of the total vitamin A. Another hindered cis-isomer, neo-c vitamin A, not so far reported from natural sources, was isolated from extracts of M. norvegica and T. raschii by fractional chromatography. It constituted about 5% of their total vitamin A. The remainder of the vitamin A was mostly in the all-trans form. The calculated biological activities of these mixtures of isomers agreed well with the results of biological assays with rats. As the vitamin A is located in the eyes of these Crustacea, it is possible that it has a visual function. The significance of these findings in relation to the vitamin A food chain in the sea is discussed, and it seems probable that the predators of the Crustacea are able to convert neo-b vitamin A into the all-trans form.