Mesodesma donacium is a marine clam distributed from Sechura Bay (5 degrees S) in northern Peru to Chiloe Island (43 degrees S) in southern Chile. Due to the commercial importance of this species, their populations have been reduced in the later years. In this work the karyotype morphology of M. donacium is described for the first time, to be additional to taxonomical, ecological, morphological, and reproductive antecedents previously documented for the species. M. donacium shows a diploid karyotype 2n = 38 with one metacentric, fifteen submetacentric and three subtelocentric chromosome pairs. The total haploid set length was 42.4 mu m, and the mean chromosome size was 2.2 mu m. M. donacium showed similitude in chromosome number with other species of the Veneroida order, but differences were found in morphology and size of the chromosomes. The cytogenetic relationships between M. donacium and other species of Veneroida is also discussed.