The karyotype of bay snook, Petenia splendida, is described based on mitotic and meiotic stages of sixty larvae and twelve juveniles from Tabasco, Mexico. Standard cytological procedures with minor modifications were followed to obtain mitotic and meiotic chromosome spreads. One hundred chromosome slides were analyzed and 290 chromosome spreads were counted. High-quality spreads in mitosis and meiosis were used for karyotype analysis. Mitotic chromosome spreads showed 76.7% of such cells with 2n=48 chromosomes, while meiotic spreads revealed 55.2% with 24 chromosomes in haploid stage. Photographic documentation of eight high-quality pictures showed that the karyotype consists of three pairs of bi-armed metacentric-submetacentric chromosomes (msm) and 21 pairs with uni-armed subtelocentric-acrocentric chromosomes (sta), with a fundamental number (FN) of 54 arms. Karyotype chromosomes were verified by analysis of haploid and diploid metaphases at meiotic stage I. Abundant chromosome spreads were observed more frequently on slides from larvae. No evidence ofheteromorphism to discriminate sexual chromosomes was detected. There were "dot-like" chromatic bodies in both sexes and they were classified as "B" chromosomes. The karyotype of P. splendida is type "A", i.e., primitive in the Cichlid family, similar to other species of Cichlasoma. The occurrence of supernumerary chromosomes is still unknown: studies on the effects of pollution and hybridization might be important to understand that phenomenon.