Non-consumptive effects (NCEs) can have substantial effects on prey populations. For example, they may alter mating interactions or interfere with foraging behavior, with potential repercussions for the entire community. Crayfish, the dominant macroinvertebrates in many temperate freshwater ecosystems, are prey to many aquatic and terrestrial species, and show behavioral responses to both predator and conspecific alarm cues. However, effects of danger cues on mating interactions have not been investigated. We exposed heterosexual pairs of Faxonius virilis to conspecific alarm cues in the mating season and compared their interactions with those of unexposed pairs. We found that almost all crayfish in both groups mated within the experimental period, but latency to begin mating varied with treatment and body size. Specifically, in the exposed group, small pairs showed increased latency to begin mating relative to larger pairs, while crayfish in the unexposed group showed the opposite pattern. Thus, crayfish responses to danger cues may be a function of their perceived risk based on body size, as smaller crayfish are likely to be more vulnerable to predators. We propose that, under natural conditions including the presence of mate competition, an increased latency to begin mating is likely to result in decreased mating success for smaller crayfish in the presence of danger cues. Such behavioral NCEs may result in differential selection on body size in environments with different predator pressure with consequent effects on crayfish populations and aquatic communities.