The aqueous extracts of thirty-four (34) tropical plants were tested in vitro for potential antimarine leech (Zeylanicobdella arugamensis) activity. The anti-leech activity was determined by exposing 8 adult leeches (Z. arugamensis) (9.3 ± 1.5 mm, aged 15 days) to 20 μl of plant extract (0.5 g/ml) for 5 min in a 24-well plate. After 5 min of exposure, the leeches were rinsed and transferred into seawater, to enable them to revive from the effect of the extract. Leech movements were observed from time to time and the numbers of paralyzed or dead leeches were recorded at 5, 20, 30 and 240 min. The efficacy of the plant extract in killing the adult Z. arugamensis during the 5 min exposure is reflected on the anti-leech property of the extract. The anti-leech property of positive plant extracts was also determined at different exposure time (1, 3, 5 min) and dilutions (1/2 (0.25 g/ml), 1/5 (0.1 g/ml) and 1/10 (0.05 g/ml). The extracts of 4 plants (Melastoma malabathricum, Piper betle, Tetracera indica and Etlingera coccinea) demonstrated anti-leech activity. The effects of M. malabathricum, P. betle and E. coccinea extracts on the leeches were very rapid causing death as early as a few seconds upon exposure. However, all four positive plant extracts were found not effective in killing the leeches at 1/10 dilution (0.05 g/ml). A Scanning Electron Microscopy examination on leeches exposed to the positive plant extracts exhibited effects such as protruding proboscis and shrunken or swollen bodies.