The objective of the work was to study behavioural change of Cyclope neritea (sea snail) and Nassarius mutabilis (land snail) upon exposure to different levels of PAHs. Snail's behaviour was translated and expressed in Behavioural State Score (BSS) where the score ranged from "0" to "5" points refers to the ascending level of locomotion of a snail. A significant difference was found in snail's behaviour in 25.0 mg/L than in 0.5 mg/L with p value smaller than 0.01. BSS scores appear most frequent on the treatment and control group were 5 (61.5-64.5%) and 2 (41.0-45.0%), respectively. Intersex behaviour was found in all species (i.e. the same sex was grouped together) regardless of PAH concentrations. This is the first reported to study the behavioural change of snail sampled in Hong Kong area when exposed to PAHs. Further studies should be carried on the impact of snail's behaviour exposure on each congener in the family of PAHs.