The diet composition and feeding habits have been investigated for several economically important indigenous species based on diel surveys collected from the Ubolratana reservoir (Thailand). For some of these populations the daily food ration, feeding periods, ingestion and evacuation rates were determined using MAXIMS, an iterative computer software utilising 24-hour series of stomach content data obtained in natural populations. The results obtained provide useful information for studies of the trophic relationship in the reservoir and can be explained by some observed migrating patterns of these fish populations.