Arsenic compounds in aqueous extracts of muscle, liver, kidney, and lung tissue taken from a sperm whale Physeter catodon (Mammalia: Cetacea) beached on a small island near Phuket in the Andaman Sea were determined by high performance liquid chromatography using an inductively coupled plasma mass spectrometer as the arsenic specific detector. The total arsenic concentrations in the tissues (dry mass) were low, ranging from 0·3 μg g−1 for liver and lung, to 1·0 μg g−1 for muscle and 3·0 μg g−1 for kidney. Most of the arsenic (>60%) was extracted from the tissue into water, and the bulk (>80%) of this arsenic was present as arsenobetaine. Dimethylarsinate was present at low levels in all four tissues whereas arsenocholine was detected in kidney, liver and lung, but not in the muscle tissue. Another arsenic containing betaine, trimethylarsoniopropionate, was also present in all four tissues. This is only the second report of trimethylarsoniopropionate as a naturally-occurring arsenic compound.