Abstract Both natural populations and laboratory cultures of the marine amphipods, Parhyale hawaiensis (Dana) and Amphithoe valida Smith, have similar isotopic relationships to their food sources. Amphipods, which ate the red alga Gelidium, had an isotopic composition identical to that of the algae, but both species of animals that consumed the green alga Ulva had an isotopic composition that differed from that of their diet by + 54 to + 70%.. The hydrogen isotopic composition of the amphipods' organic tissue was not related to that of the surrounding water, because the isotopic composition did not change when the animals were exposed to water enriched in deuterium. The algal food sources that grew in waters of fairly uniform deuterium-hydrogen composition showed no apparent seasonal variations for the incorporation of deuterium. These same algae, in particular Ulva sp., undergo decay exchange hydrogen to resemble more closely surrounding waters. The organically bonded hydrogen in amphipods consuming the decaying plantmatter followed this change sympathetically. Stable hydrogen isotopes can be used in a similar fashion asnitrogen and carbon isotopes in the resolution of complex foodwebs.