Several microanalytical techniques were used to assess the distribution and chemical form of chromium in various tissues of Mytjlus edulls, at the cellular and subcellular levels After 2 wk exposure to Cr (111) salt (Cr occurs principally in the trivalent state in the natural environment), investigations were performed on mussels, using secondary ion mass spectrometry (ion microscope and ion microprobe) associated with photon microscope, and X-ray spectrometry (electron microprobe) associated with transmission electron miscroscope. Cr measurements gave the following: kidney and gills exhibited the highest values, intermediate values were found for muscle and byssus (where Cr was adsorbed onto the threads and was incorporated within them), and lowest values were found in the digestive gland. Cr was also detected in the amoebocytes but not in the reproductive cells. The target organelle of Cr accumulation was shown to be the lysosome where the metal was associated with phosphorus and sulfur and trapped in an insoluble form. The significance of these different data is discussed. Cr kinetics of metabolism was compared between mussels and other aq.uatic organisms. Comparison in M. edulis of Cr metabolism with metabolism of other metals leads to the conclusion that Cr is metabolised and transported differently from most toxic metals.