1. Chromosome numbers from 12 species of Enchytraeidae representing 7 genera are reported. The following numbers were found n: 13, 14, 15, 16, 17, 21, 32 and 2n: 120–140 and 180–200. 2. Several cases of intraspecific chromosome variation are found, and both intra- and interspecific polyploidy has been observed. 3. The meiotic behaviour of the chromosomes in several polyploid species, and the absence of spermatozoa in one sexually mature species suggest parthenogenetic reproduction. Breakdown of sexual reproduction seems to be associated with polyploidy. 4. The taxonomical implications of the results are discussed in relation to Muldal's investigations of the Lumbricidae and his hypothesis as to the evolution of chromosome numbers in Oligochaeta.