We investigated the effect of the PLAG1 gene (on bovine chromosome 14) polymorphism on oleic acid percentage (C18:1) in four Japanese Black cattle populations (JB1: n = 900, JB2: n = 560, JB3: n = 456, JB4: n = 450). We genotyped the PLAG1 polymorphism (rs109231213) for four populations and then analyzed the effects on carcass weight (CW) and C18:1. This polymorphism was significantly associated with CW in these four populations (P<0.001), in agreement with previous reports. In addition, it also had a significant effect on C18:1 in JB1, JB2 and JB3 (P<0.05). However, considering that the effect of PLAG1 differed among populations, it was suggested that the PLAG1 polymorphism would not have a direct effect on C18:1 and would be in linkage disequilibrium (LD) with a causative mutation. Turkey-Kramer's honestly significant difference test revealed that the qq genotype had a higher percentage of C18:1 than the QQ genotype in JB1 and JB2 (1.62 and 2.23, respectively), while the qq genotype showed lower CW in all four populations. These results suggest that the PLAG1 gene polymorphism would be useful as a DNA marker for C18:1 in Japanese Black cattle populations; this H. Kigoshi et al. 2 information could contribute to the identification of a causative mutation on BTA14 for fatty acid composition.