<p>The rice blast disease, caused by <i>Pyricularia oryzae,</i> poses a significant threat to rice production worldwide. The resistant rice varieties often fail to maintain persistence in fields. The emergence of fast evolving races is attributed to defeating newly released rice resistant varieties. Vegetative compatibility has been a useful tool in assessing genetic diversity in ascomycetous fungi. In order to investigate vegetative compatibility among <i>P. oryzae</i> isolates in Taiwan, nitrate non-utilizing (<i>nit</i>) mutants were recovered from 87 isolates for analysis. Additionally, sulfate non-utilizing (<i>sul</i>) mutants were also recovered from isolates, showing no complementation with their respective <i>nit</i> mutants. A high percentage (35.19 %) of heterokaryon self-incompatible (HSI) isolates was revealed, as indicated by the lack of complementation in pairings of their <i>nit</i> / <i>nit</i> mutants and <i>nit</i> / <i>sul</i> mutants. When isolate 63 was used as a tester isolate in pairings with other 86 isolates, 78.4% of the 86 isolates formed weak to strong heterokaryon formations to the tester isolate, suggesting that these isolates were a putative vegetative compatible group. Unexpectedly, further assays displayed that isolates forming the weak type of heterokaryon formation with the tester isolate did not fuse to each other, and heterokaryon formation might not occur between isolates forming the strong type of heterokaryon formation with the tester isolate. The findings unveil the heterogeneity within the vegetative compatibility group, suggesting implications for race diversity and the intricate regulatory mechanisms of vegetative compatibility in <i>P. oryzae</i>, as discussed.</p>