The freshwater Amphipod Gammarus pulex is a sensitive indicator organism for environmental contamination. In this study, effects of heavy metal lead acetate were investigated by using electron microscopy techniques. Control group was not expose to lead acetate but experimental group was for 96 h. The ultrastructural changes of hepatopancreatic ceca cells were studied in these two groups by electron microscopy techniques analysing cellular structure, structure of organelles and vacuolization. The number of cells based on the toxic effects of lead acetate was increased compared with the control group. The number and length of cell microvilli was decreased. Additionally degenerated mitocondria, dilation of golgi vesicules and cytoplasmic vacuolization were observed. The results of present study suggest that the exposure to the lead acetate may cause some Ultrastructural changes on hepatopancreatic ceca of digestive system in Gammarus pulex. The significance of this change in terms of environmental toxicology needs to be further studied. (C) 2002 Elsevier Science B.V. All rights reserved.