The air-breathing fish Channa striata is widely distributed in South-East Asia, and is an important source of protein for local inhabitants. The fish can be found in various water bodies including lakes, reservoirs, ponds, canals, drains and rice fields where agro-chemicals are applied. This species has been reported to die if surface breathing is prevented. The distinctive surfacing (where the fish swallow air from the surface) and hanging (where the fish remain motionless just below the surface) behaviors in this species depend on the dissolved oxygen level. Surfacing frequency was negatively correlated with the partial pressure of oxygen (PO2) but hanging behavior was positively correlated with PO2. Thus, although this fish is an obligate air breather, it can alter its respiratory strategy depending on the dissolved oxygen concentration (DO) in the water. Increased air-breathing activity and reduced oxygen uptake over the gill have been found when C. striata was exposed to metasystox at the concentration causing 50% mortality during 48 hours of exposure (LC50-48 h). This effect of toxins on surfacing behavior has also been shown in another species of air-breathing fish,Macropodus cupanus, which increases surfacing frequency with increasing sublethal concentrations of carbaryl. This shift can be viewed in terms of chemical avoidance since the gills are often the most important route of chemical uptake in fish. Swimming down concentration gradients is the only possibility for toxin avoidance in fish species without the ability to breathe air, but air-breathing fishmay be able to avoid the toxin by shifting the source of oxygen from contaminated water to air. Diazinon is one of the most commonly used insecticides in rice fields of the Mekong River Delta (MRD). Although diazinon is not persistent in the environment, it is highly toxic to vertebrates including fish. Diazinon is a neurotoxin through its ability to inhibit acetylcholinesterase, causing abnormal swimming behavior and altered respiration and social interactions in aquatic organisms. Water contamination with insecticides occurs mostly at concentrations that do not cause massmortality, and sublethal effects are bothmore widespread and more important for population dynamics. This study aimed to determine whether low sublethal concentrations of the organophosphate diazinon, which the fish may encounter in its natural habitat (0.008 and 0.079 mg/L), caused rapid alterations in behaviors related to respiratory strategy, particularly surfacing and hanging. Adult C. striata were obtained from a laboratory stock population (Can Tho University, Vietnam) at a size of 40.3 3.5 g (mean standard deviation). The fish had been raised from larvae in chlorinefree tap water (pH 7.60 0.03, DO 7.9 0.1 mg/L, water temperature 26.7 0.4°C) for one year. The commercial insecticide Basudin 50EC, containing 50% wt/wt [6-methyl-2-(1-methylethyl)-4pyrimidinyl] ester, common name diazinon, was purchased (Angiang Plant Protection and Services Co., Long Xuyen City, Vietnam). The insecticide *Corresponding author: Tel: 84-71-83-5701. Fax: 84-71-83-0323. Email: ntphuong@ctu.edu.vn Received 13 March 2006. Accepted 17 August 2006. FISHERIES SCIENCE 2008; 74: 1330–1332