SUMMARY: A new variety of Paramecium aurelia-variety g-containing two mating types XVII and XVIII, has been identified from seven samples of water collected in Yotland. Variety 9 belongs to the group A varieties in the Sonneborn system of classification, but shows no intervarietal mating reactions. Paramecium aurelia is readily distinguished from other species of Paramecium by morphological characters, such as the structure and number of micronuclei (see Wenrich, 1928). Within the species P. aurelia, however, no fewer than eight distinct varieties or physiological subspecies have been recognized (Sonneborn, 1938, 1942; Sonneborn & Dippell, 1946a, b). The organisms are classified into one or other of these varieties according to mating behaviour: within a variety, conjugation may be readily brought about between the two mating types which occur in all varieties (except no. 7), and leads to the production of viable progeny; between varieties, by contrast, conjugation may not occur at all, or if it does, the Fl hybrids, or the F, progeny derived from them, are non-viable. Recently we have collected from a number of localities in Scotland samples of paramecia which have been identified as Paramecium aurelia from the two small vesicular micronuclei seen to be present in stained preparations. The