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The effectiveness of selection for one trait, low backfat thickness, was investigated in Poland China pigs. Backfat probe measurements were made at a mean weight of 79.2 ± 2.5 kg. Measurements were made at three sites, shoulder, loin and ham and the average of the three measurements was used for selection. Mass selection resulted in approximately a 20 percent decrease in the backfat thickness of pigs as measured by the probe technique in five generations of selection. Response to selection at the three probe sites was similar. Realized heritability for the average of the three backfat probes was 0.32±0.09, while the heritability estimate obtained from intrasire regression of mean of offspring on dam was 0.56±0.09. The genetic correlations between backfat probes at 3 sites ranged from 0.59±0.10 to 0.82±0.06, suggesting that many of the same genes affected backfat at the different sites. The results of this study showed that mass selection for low backfat thickness was an effective method for reduction of backfat thickness.