This study investigated the spatial and temporal distribution in species composition and numerical abundance of thaliaceans (Tunicata) in relation to environmental factors in the waters off southern Taiwan between February 2004 and October 2005. In total, 18 thaliacean taxa belonging to 10 genera and 3 families were identified, with the mean abundance of 152 ¡Ó 33 ind./100m3. The abundance of thaliaceans showed clear seasonal change, higher in winter. The five most dominant species were Thalia rhomboides, Pyrosomella verticillata, Doliolum denticulatum, Thalia orientalis and Thalia democratica. They were widespread oceanic species, and together constituted 82 % of the total thaliaceans. The abundance of thaliaceans showed significant negative correlation with temperature. Different predominant thaliacean species showed different correlationship with environmental factors; Thalia rhomboides, Doliolum denticulatum and Thalia orientalis were significant and negatively correlated with temperature. Result of ANOVA test revealed that the abundance of thaliaceans and hydrography had significant seasonal differences in the waters off southern Taiwan, but showed no significant difference among stations. The variation of dominant thaliacean species composition among the water masses was small, implied that the influence of water masses on the distribution of thaliaceans was minor.