A bioassay for determining the effects of toxicants on the growth of a sensitive life stage of Gammarus pulex (L.) is described. The experimental design minimised variation: a non-destructive weighing technique produced initial treatment groups with a low coefficient of variation for mean wet weight and also facilitated the introduction of a standard artificial diet regime. The chemical investigated was the organochlorine insecticide lindane for which the lowest observed (LOEC) and no observed effect concentrations (NOEC) of 6.1 and 2.7 mu gl(-1) respectively were determined after the 14 day exposure period. These results are compared to toxicity data from other studies. (C) 1996 Elsevier Science Ltd