The geographical location of Taiwan has rendered to her high marine biodiversity. One of the objective of the present study is to survey the nudibranchia in Taiwan and her adjacent islands in including the Liu-chiu Yu, Peng-hu, Green island and Orchid island (Lan-yu) and the related zoogeographic issues. Field surveys resulted in 60 nudibranch species on the northeastern coast, 42 species on Nanwan (Taiwan), 43 species on Peng-hu, 33 species on Liu-Chiu Yu, 29 species on Green island, and 18 species on Orchid island (Lan-yu). Among these areas ,species diversity peaked on the northeastern coast. Analysis of species component led to two clusters, the northeastern coast-Peng-hu and Liu-chiu Yu- Nanwan-Green island- Orchid island(Lan-yu). An altitudinal effect may account for that similarity pattern. Results also indicated that Phyllidiella pustulosa is the top dominant species in almost all the surveyed areas. Upon subjected to external stimulation, P. pustulosa responded by excreting a white mucous. A second objective of the present study is to examine the cell or structures responsible for the secretion of this possible poisonous mucous. Okenia japonica is an unusual species found only on Peng-hu. In the spring of 2004, it was found at Long-men in an exceptional by large number of as many as 87 individuals and these small nudibranchs were associated with a species of block sponge. Both this nudibranch species and the sponge were absent at in the summer of this year. They recurred in the spring of 2005 but being not as abundant as in the previous spring. Appearance of this nudibranch may be closely related to the sponge. Among all the surveyed sites, An Wai-an on Pen-hu was the top species site: 15 species in the spring of 2005, 20 species and 12 species in the spring and summer of 2006, respectively. Most of them were Chromodorididae. Factors leading to this ¡¥hot spot¡¦ phenomenon remains to be studied. Among the sites on Liu-Chiu Yu, Meiren-dong were the most species site. Seasonally, species number at this site peaked in the fall. Phyllidiidae species were most commonly on Liu-chiu Yu, Green island and Orchid island (Lan-yu). Instead, Chromodorididae species were most common on Pen-hu. No obvious dominant group was found on the northeastern coast. The predominant muddy seafloor on the Pen-hu may provide a proper habited for Chromodorididae. Cells associated with the white mucous of P. pustulosa distributed in underneath the external layer of the mantle in. Extraction of this mucous showed lethal effect on cancer cells. Consequently, this mucous is poisonous. Its capability of chemical defense may have granted its dominance in these surveyed areas.