Twenty-eight macrodasyid and twenty-eight chaetonotid species of marin
e Gastrotricha are reported. Of these species, two Halichaetonotus cla
vicornis and H. genatus - are new, and three are described for the fir
st time for to the Italian fauna. The influence of various physical an
d chemical environmental factors on gastrotrich distribution along the
peninsular and island coasts of the Tyrrhenian Sea are analysed and d
iscussed.