Evaluation of spatial-temporal variability of species composition and diversity in oceanic ecosystems is not easy because it is usually difficult to obtain sufficient data quantifying such variability.In this study,we examined pelagic species diversity indicators,species richness,Shannon-Wiener index of diversity and Hurlbert’s species evenness,for fish assemblages from two areas(north and south) in the North Pacific Ocean(2°±12°N,178°E±165°W) during May±July 2008.The assemblages were based on data collected by an onboard scientific observer during a commercial longline fishing trip.The species richness and Shannon-Wiener diversity index of fish assemblages in the northern area were slightly higher than those in the southern area,although these differences were not significant(t test,P.0.05).Non-parametric multidimensional scaling and analysis of similarities indicated that there were significant differences in fish assemblages between the two areas(P,0.01). Evaluation of spatial-temporal variability of species composition and diversity in oceanic ecosystems is not easy because it is usually difficult to obtain sufficient data quantifying such variability. In this study, we examined pelagic species diversity indicators, species richness, Shannon-Wiener index of diversity and Hurlbert’s species evenness, for fish assemblages from two areas (north and south) in the North Pacific Ocean (2 ° ± 12 ° N, 178 ° E ± 165 ° W) during May ± July 2008. The assemblages were based on data collected by an onboard scientific observer during a commercial longline fishing trip. species species richness and Shannon-Wiener diversity index of fish assemblages in the northern area were slightly higher than those in the southern area, although these differences were not significant (t test, P. 0.05) .Non-parametric multidimensional scaling and analysis of similarities that there were significant differences in fish assemblages between the two areas (P, 0.01).