Sexual dimorphism and the reliable identification of sex based on external characteristics are important for ecological, biological, evolutionary, and many other fields of animal research, including studies on amphibious fishes. This study examined morphological and meristic variation in 135 individuals of the mudskipper Scartelaos histophorus from the Mekong Delta, Vietnam. Sex was determined by gonadal inspection and compared with external genital papilla morphology, morphometrics, and meristic traits, including fin-ray counts. Females exhibited a rounded, dark red genital papilla, whereas males showed an elongated, pale papilla with a pointed tip. These consistent differences suggest that genital papilla morphology provides the most practical and reliable means of sex identification in this species. Unadjusted morphometric comparisons suggested sex-related variation in first dorsal fin length and body depth; however, analysis of covariance (ANCOVA) indicated that the difference in first dorsal fin length was not retained after controlling for total length and sampling site. A linear discriminant analysis based on total length, first dorsal fin length, and body depth provided only moderate sex-classification performance, indicating that morphometric measurements are supplementary rather than primary diagnostic criteria. Notably, first dorsal fin-ray (FiDR) counts displayed clear sexual dimorphism, with males typically possessing four rays and females five. This trait enabled sex classification with sex-specific accuracies of 81.6% in males and 90.7% in females, and an overall accuracy of 87.4%. These findings provide a practical basis for rapid, noninvasive sex identification in field conditions and contribute to understanding patterns of sexual dimorphism in oxudercid fishes.

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