Regular quantitative analysis of water quality (basic, elemental and pesticides) including seasonal and diurnal fluctuations was conducted using standard techniques between January 1988 and December 1988 and between May 1989 and April 1990 at an irrigated rice-fish pond in Munoz, Nueva Ecija, Luzon Island in the Philippines, an area where the epizootic ulcerative syndrome (EUS) had been predicted. Agriculture/aquaculture management practices and climatic conditions were closely monitored and quantitative sampling of fish populations was undertaken over the two-year period. The occurrence of EUS from the study site was recorded in March, June and between September and November 1988 and between October 1989 and March 1990. Of the seven species of fish sampled, only Ophicephalus striatus, Clarias batrachus and Trichogaster pectoralis were affected. The prevalence of EUS was highest in O. striatus - 15.4% (23/149). The significant environmental variables in relation to EUS include temperature and rainfall, although there were recorded low levels of some chemical water quality variables such as chloride and hardness at the time of the outbreaks. Seasonal and diurnal fluctuations in most of the water quality variables measured were evident at periods before and during the outbreak of EUS. Pesticides and heavy metals do not seem to have a direct influence on the occurrence of EUS at this particular location. The EUS-affected snakehead haboured Trichodina, Epistylis and Chilodonella. Bacterial isolates from kidney of EUS-affected snakehead were identified as Aeromonas hydrophila, Flexibacter columnaris and members of the Family Enterobacteriaceae. Histopathological observations showed a diffuse infiltartion of non-septate fungal hyphae, a large number of inflammatory cells through the lesions, sarcolysis of muscle fibres, oedema and hyperplasia of gill lamella and minimal changes in the internal organs.