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Abstract. Aggressive behaviors are described for five species of the genus Ilyoplax (Dotillidae), I. pusilla (De Haan, 1835), I. formosensis (Rathbun, 1921), I. ningpoensis Shen, 1940, I. deschampsi (Rathbun, 1913, and I. tansuiensis Sakai, 1939, and the male/female ratios are compared relative to the sex ratio of the population. Aggressive behaviors were similar in all species. The main types of aggressive behaviors, including aggressive dash, aggressive wave, and wrestling, were common to the five species. The mutual aggression of wrestling was confined to male crabs in all five species. The frequencies of aggressive behaviors were higher in male crabs than in female crabs, suggesting male-biased aggressiveness. This may be reflected in the male-biased carapace and chela size dimorphisms observed in I. pusilla.