Mesodinium rubrum is a marine planktonic holotrich ciliate of extremely wide geographical distribution which has often been found to be associated with ‘red water’ phenomena (Taylor, Blackbourn & Blackbourn, 1971). It has mainly been recorded from extremely neritic locations such as bays and fjords; away from the coast it is usually associated with regions of upwelling and in such conditions the blooms have been recorded as extending over areas as large as 100 square miles (Ryther, 1967). Fifty instances of the occurrence of M. rubrum are usefully summarized by Taylor et al. (1971).