Male fiddler crabs either produce lateral or vertical waves. The function of lateral waves is well documented, but vertical waves have been largely unexplored. Here we explore the function of vertical claw-waving by the Japanese fiddler crab Uca arcuata by examining the context in which males produced both directed waves (the waver is facing a nearby crab) and undirected waves (waving when no other crabs are present). There are two types of mating in this species: burrow mating (the male chases a female to his burrow and they mate underground); and surface mating (the male holds the female down and they mate on the surface). Attempted burrow matings were more likely to be preceded with directed waving at the female than were attempted surface matings (83% vs. 50%). All successful burrow matings were preceded by directed waving at the female, while only 50% of successful surface matings were preceded by directed waving. Undirected waving was given in the absence of a nearby crab. The frequency of undirected waving was not related to the number of males within a 60 cm radius around the male, but was negatively related to the number of females within that distance. This suggests that undirected waving acts as a long-range female attractant.