Measurement and dissection of 97 museum specimens provided information on morphology, food habits, and reproduction in the colubrid snake Symphimus mayae. This medium-sized (<910 mm total length) diurnal species from the Yucatán Peninsula is an inhabitant of lowland xerophytic habitats, and an orthopteran dietary specialist. Sexual dimorphism is evident in relative tail length, head length, and numbers of subcaudal scales, but not in the number of ventrals, and males and females attain similar adult body sizes. Symphimus mayae feeds almost exclusively on orthopteroid insects, principally katydids (Tettigoniidae) and crickets (Gryllidae). These taxa jointly accounted for >50% of the total number of prey items recovered from stomachs and had been eaten by >70% of the snakes examined, with little intraspecific or seasonal variation. Other, less frequently used prey groups are (in order of relative importance), stick insects, mantids, Jerusalem crickets, grasshoppers and spiders. All size classes of snakes eat orthopterans, but stick insects are consumed predominantly by adults (>300 mm SVL), and mantids exclusively by large adults (>420 mm SVL). More than 55% of individuals from monthly samples contained recently ingested food, suggesting that feeding in S. mayae occurs year-round and is not significantly curtailed by the local dry season. Reproduction is probably annual, although it does not occur in all females every year, and a single clutch of two to four eggs is produced with oviposition and hatching timed to the wet season.