Using three-dimensional (3D) video observations in laboratory experiments, I describe interspecific and intergeneric mating behaviors and motility patterns of the common planktonic marine copepods Centropages typicus, Centropages hamatus, and Temora longicornis. These observations are then used to estimate heterospecific and conspecific male mate-search volume rates and mate encounter rates in North Sea Centropages populations. Behavioral prezygotic reproductive isolation between Centropages species is incomplete, since males of each species pursued, contacted, captured, and, in rare cases, placed a spermatophore on the urosome of heterospecific females. T. longicornis males also detected the diffusible pheromone trail and pursued C. typicus females to the point of mate contact. Male mate-search tracking behavior was equally effective on diffusible pheromone trails of heterospecific and conspecific females, indicating that pheromone and hydromechanical precontact mating cues lack species specificity. Males attempted mating with both heterospecific and conspecific females at high frequencies in the laboratory, and species recognition is inferred to occur through contact chemosensory cues or morphological incompatibility between species. Heterospecific mate encounter rates in the North Sea were maximal in late summer (August), up to,2,000 encounters m(-3) d(-1), and were comparable in magnitude to conspecific encounter rates during the same period. C. typicus females experience the highest incidence of heterospecific mating interactions, since they encounter heterospecific males at rates up to 100+ encounters female(-1) d(-1), ca. one order of magnitude higher than encounter rates with conspecifics. Heterospecific mating attempts may be a common feature of the reproductive ecology of planktonic copepods and may incur substantial fitness costs to the individuals involved.