Nineteen Eastern Australian populations of the grasshopper Atractomorpha similis (Acridoidea, Pyrgomorphidae) were sampled and male meiotic chromosomes, as well as some male and female somatic mitoses, were examined. In fourteen of these populations, a proportion of the males were found to carry between one and ten extra copies of a particular autosome, the megameric chromosome (A9). Numbers of extra chromosomes varied between but not within the individual follicles of the testis. The extra chromosomes were not found in somatic tissue. In all, 20% of males from the field were germ-line polysomic and within these males, 91% of germ cells were polysomic. In meiosis, extra copies of A9 present as univalents lagged at anaphase I or II and subsequently formed micronuclei which degenerated early in spermiogenesis. As one extra univalent is the most common polysomic condition in natural populations, this elimination of univalents suggests that most polysomic males produce a large proportion of normal haploid sperm. In laboratory cultures, selection for increased frequency of germ-line polysomy, conducted over four generations, raised the proportion of polysomic males from 23% to 71%. Selection against polysomy reduced its frequency to 5%. These breeding experiments also showed that germ-line polysomy is equally transmissible through both the male and the female parent. Transmission data also suggested that these extra chromosomes can arise de novo, presumably by unequal disjunction in previously diploid lines. A computer model was devised, simulating the effects of repeated non-disjunction over a series of mitotic divisions. The behaviour of this model suggested that the distributions of extra chromosome numbers observed in the laboratory generations most probably resulted from such a series of non-disjunctions, occurring in an initially diploid cell population. It seems, therefore, that the transmission of polysomy occurs through the agency of heritable factors which determine the probability of non-disjunction and thus the accumulation of a particular autosome during a specific series of mitotic divisions in the embryonic germ-line.