ABSTRACT: Diel vertical migration of Thaliacea was monitored during different seasonal cruises in 1997 in the southwestern waters of Taiwan using samples collected by multiple plankton sampling nets and a 1 m plankton net. Overall, Doliolumdenticulatum, Thalia democratica and T. orientalis were the 3 most dominant species in all seasons, and contributed ≥ 93% of the numerical total of Thaliacea. These species exhibited about a 2 times higher abundance at night than during theday, if all the data were integrated. In the seasonal multiple net samples, D. denticulatum and T. democratica exhibited an alternation between sexual and asexual reproduction between different sampling seasons, while aggregate zooids ofT. orientalis, when compared with solitary zooids, dominated in all seasons and apparently increased from July to December. Different stages of these dominant species had different preferences for water depth in different seasons, suggesting thatthe vertical distribution of these species might be related to reproduction or food. The abundances of different stages of these thaliaceans were correlated with temperature and salinity. The time-series samples of surface water showed that speciesrichness and abundance of these species varied seasonally, but was always greater at night than during the day.