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Does an animal's reproductive state influence the distance at which it flees from an approaching predator? We predicted that reproduction would increase approach distances in pregnant females (because they are burdened with eggs and thus less able to escape rapidly), but reduce them in males (because of lower vigilance due to males focusing on mate-searching rather than predator detection). Field data on approach distances of keelback snakes (Tropidonophis mairii, Natricinae, Colubridae) supported both of these predictions. We walked the same 1.3-km transect along a dam wall in tropical Australia on 135 nights, and recorded the distances at which snakes fled from our approach. Locomotor speeds were measured for a subset of these animals. Variations in approach distance due to season, weather conditions, prior capture history and snake body size were minor, but reproduction strongly affected approach distances for snakes of both sexes. Gravid females were slower than non-gravid females, and fled at greater distances. Reproductive status did not affect locomotor speeds of males, but males that were reproductive (i.e., contained sperm) permitted closer approach than did non-reproductive adult males. Reproduction thus affected approach distances in snakes of both sexes, but in opposite directions and for different reasons.