Extensive karyotype variation was found in both somatic and meiotic cells of the progeny in a laboratory-reared family of the loach Misgurnus anguillicaudatus. Only one of seven individuals examined showed the standard karyotype, 2n = 50 comprising 10 metacentrics, 4 submetacentrics and 36 telocentrics. However, the other six individuals exhibited hyperdiploid karyotypes due to the presence of additional telocentrics and micro-chromosomes, resulting in chromosome numbers 2n = 51 to 53, or 58 plus 0 to 5 micro-chromosomes within or between individuals. Such inter- and/or intra-individual variation in chromosome number was also observed in primary spermatocytes of three males with an increase of bivalents and univalents involving additional telocentrics, although micro-chromosomes were seldom paired. Accumulation of additional chromosomes was apparent in spermatogonia and spermatocytes with greater incidence than in somatic cells. Euploid and aneuploid loaches were not discriminated by the external morphology, suggesting the observed additional chromosomes to be genetically neutral.