Optimal sample size methodology, which has been used by plant and marine ecologists for decades, is applied in this study to whole-community invertebrate data in the form of a case study. Wiegert’s method, based on maximizing the precision of biomass or catch data for the least cost in sampling or processing time, was conducted on guilds, habitat associations, and entire communities of tropical rice invertebrates at three crop stages (vegetative, reproductive, ripening) using five circular quadrats (35, 45, 55, 65, 80 cm in diameter) and a petrol-driven suction apparatus. Analysis of invertebrate samples and comparisons of different quadrat sizes showed that: (1) invertebrate abundance was a better predictor of processing time than was either species richness or species evenness for two of three crop stages; (2) average processing time per sample increased with crop age; and (3) size recommendations for one crop stage did not correspond to another for the same invertebrate group; however, after averaging over crop stages, one optimal quadrat size emerged for three out of six faunal groups. Unexpectedly, the optimal (and most practical) quadrat size determined for rice-invertebrate communities (35 cm diameter) was smaller than the majority (90%) of published studies have previously used, leading us to conclude that the majority of researchers (including the present authors) have used larger quadrats than were needed to obtain the same precision. Results of this case study underscore the need for researchers to conduct efficiency tests of their sampling program before starting large field trials and to resist the temptation of using published quadrat sizes whose optimality characteristics have not been field tested.