Northern quahogs (=hard clam; Mercenaria mercenaria) is a commercially important bivalve naturally distributed along the American Atlantic coast. Because the hard clam was first introduced into China as a candidate for aquaculture in 1997, research has been conducted on its ecologic habits, ecological physiology, metabolism, growth, reproduction, artificial propagation, and seed culture. Little information about their chromosomes and karyotype is available. The present study is designed to study the karyotype of the diploids and triploids to provide a reference to genetic research on introduced M. mercenaria. Triploids were produced by inhibiting the release of second polar body (PBII) with a treatment of 450 μmol/mL 6-deimethylaminopurine (6-DMAP), with untreated fertilized eggs developing as normal diploid controls. Chromosome preparations were made from trochophore larvae with drop-splash method. Chromosomes were measured and classified according to Leven criteria. Statistical analysis of relative length and arm ratio revealed that, this species had a normal diploid chromosome number of 2n = 38, and the karyotype consists of 15 pairs of metacentric (M) and 4 pairs of submetacentric (SM) chromosomes, NF = 76. The triploid karyotype formula was 45 M 12 SM, NF = 114. The heterochromosomes and satellite chromosomes were not observed in all chromosomes in the complement.