Sperm of 28 boars of the Landrace breed was evaluated with the help of the thermoresistance test (TRT). The principle of the TRT is that the sample of fresh sperm is left in a 38 degrees C bath for 120 min and after that time the activity of the sperm is evaluated. 308 ejaculates were evaluated in this way and 1342 sows were inseminated by these ejaculates after a short storage. The highest conception rate was stated after insemination with the sperm whose activity was above 50% at the end of the TRT. 366 of 400 sows became pregnant, i.e. 82.43%, whereas out of 370 sows inseminated with sperm whose activity was 0-30% at the end of the TRT only 269 conceived, i.e. 72.70%; difference was statistically highly significant. No statistically significant difference was recorded between the fertility (number of piglets per one litter) and the TRT results. A significant dependence was observed between the sperm survival after 72 hours and conception (P less than 0.05). The same significant relation was found between survival within 72 hours and TRT in fresh ejaculates. This justified the recommendation to introduce the TRT among the test used for evaluation of the quality of fresh boar ejaculate.