[show abstract][hide abstract] ABSTRACT: Chromosomal imbalance in gametes and embryos is one of the factors contributing to early embryonic mortality. Although the rate of chromosomally abnormal sperm cells is low and usually does not exceed 1%, there is no clear indication of fertilizing potential of such gametes. The aim of the experiment was to investigate the type and incidence of numerical chromosomal aberrations in spermatozoa produced by fertile boars used in artificial insemination (AI). We used the protocol of fluorescent in situ hybridization (FISH) on sperm interphase nuclei with molecular probes for porcine chromosome pairs 1 and 10. Altogether 12 348 sperm cells were examined. Disomy was observed in spermatozoa of all seven AI boars whereas only one diploid cell was identified in all screened sperm cells. The average rate of chromosomally unbalanced sperm was 0.105% (13/12 348) with an inter-individual variation from 0.048% to 0.194%. Among abnormal sperm cells, both disomy (0.097%) and diploidy (0.008%) were detected. Nullisomy was not included into calculations. The estimated aneuploidy rate calculated by doubling the number of disomic cells was 0.194%. Chromosome pair 10 was significantly more often involved in non-disjunction (75%, 9/12 aneuploid sperm cells) than chromosome pair 1 (25%, 3/12). We have shown for the pig that the rate of disomic cells falls into a range presented by other authors, whereas that of diploid spermatozoa appeared to be lower in the present study. In conclusion, numerical chromosome aberrations were present in spermatozoa of all AI boars analyzed in this study. Therefore, it can be assumed that the presence of unbalanced spermatozoa at the level observed in fertile males does not significantly affect their reproductive potential.