The biology of the marine leechHemibdella soleaewas studied. It was found that multiple infections were common, the leeches being attached by the posterior sucker to spines on the ctenoid scales of the upper surface of the sole. It was observed that in aqqaria they change position every few days, tending to migrate antoriorly and this orientation may perhaps be directed by the arrangement of the scales.Cocoons, which are not deposited at 8d̀ C or below, are attached to the shell fragments with which the soles camouflage themselves and the young leeches hatch after about forty‐one days at 17d̀ C. Leeches reach maturity about twenty‐three days after finding a host and are fully grown after a further fortnight.BritishHemibdellaseem to be confuned toSoleu soleaalthough several other hosts (All Soleidae) have been recorded for this leech in the Mediterranean.