The mason bee, which belongs to the Megachilidae family is one of the wild-living bees which has been used for many years to pollinate early-spring orchard cultivations and isolated seed plantations. The aim of this study was to determine the number of females which a single mason bee male is capable to inseminate in laboratory conditions. The experimental material comprised males (30 individuals) and females (165 individuals) of mason bee. The trial included 10 experimental groups differing with regard to the number of females (1 to 10) at the same number of males - 1 for each group. Each group had three repetitions. The insects were kept in individual boxes to allow them to copulate. Next, the analysis of female prepared seminal receptacles was performed during which their size and the number of sperm cells found inside them were determined. Chi test - square, Duncan test and one-way analysis of variance were employed in the performed statistical calculations. Different numbers of inseminated females were observed in each experimental group. The performed experiment revealed that the best insemination results of females were recorded in the experimental group in which there was one male for three mason bee females and the maximum number of inseminated females by a single male was five. The mean volume of seminal receptacles in the inseminated females was slightly bigger than in non-inseminated females, although this difference was statistically non-significant. The mean number of sperm cells in the seminal receptacles of the inseminated females ranged from 1400 to 5683. The performed analysis of variance showed that there were no statistically significant differences between the number of sperm cells in seminal receptacles and the number of inseminated females in individual combinations.