AbstractIn many parts of the developing world, leachate treatment system is not properly installed and managed that landfill leachate usually flows into existing water bodies. Consequently, heavy metal in leachate could lead to biomagnification of heavy metals as the components travel higher within the food chain. Thus, it is necessary to understand the characteristics of leachate and its toxicity potential prior to its release to the water bodies. This paper aims to determine the toxicity effect of different types of landfill leachate on Anabas testudineus. Also, the behavioural changes towards leachate toxicity will be recorded. To achieve the objectives, toxicity tests were conducted on A. testudineus. It involved three main stages namely acclimatisation, range finding test, and short-term definitive test. For the purpose of the study, leachate samples were collected from an active sanitary landfills and a closed sanitary landfill. Ten A. testudineus with average weight of 4.2g and average length of 4.0cm were introduced into leachate with five different concentrations ranging from 3.125% to 5.625%. The mortality rate was observed and recorded after 96hours exposure. The LC50 of A. testudineus was calculated using EPA Probit software. Leachate from non-active landfill is alkaline (pH 8.2) as compared to that of the active landfill (pH 7.35). As expected, COD from the closed landfill is much lower (10,000mg/l) than the active landfill (24,800mg/l) while the BOD5 was 3,500mg/l and 12,500mg/l, respectively. However, the result of ammoniacal nitrogen was highly different between these landfills, where closed landfill is releasing higher concentration of ammoniacal nitrogen (880mg/l) than the active landfill (0.085mg/l). The toxicity test indicated that both landfill leachates are highly toxic to A. testudineus. In fact, leachate from the closed landfill was more toxic than leachate from the active landfill. Results indicated that the LC50 of the leachate from closed landfill on A. testudineus was 4.71% v/v while LC50 of the leachate from active landfill was 5.1%. Discolouration of exposed fishes was observed in the study, and it could be due to the loss of colouring pigment caused by ammonia poisoning. Other observation of behavioural changes included swimming disorder, loss of equilibrium, unusual leaping action, and declination in the general activities. These are most probably due to neurotoxin effect inflected by the leachate exposure. In conclusion, leachate from active and non- active landfills is toxic to A. testudineus based on the behavioural changes and the high mortality rate