Tin is reduced to stannane by sodium tetrahydroborate solution, passed through a heated silica tube and measured by atomic absorption specirometry at 286.33 nm. The detection limit is 0.9 ng ml -1 and the r.s.d. is 4.1% at 0.01 μg ml -1. Interferences are discussed and the sample decomposition procedure evaluated. Typical results obtained for macroalgae. fish and molluscs are given.