1. Territorial male threespine sticklebacks, Gasterosteus aculeatus, do not always attack an intruder (a dummy) with red undersides more than one lacking this feature even when it is presented within their own territory. Males in this study showed an overall tendency to bite a dummy without red undersides more than one with this feature during both simultaneous and serial presentations. 2. Considerable variation in attack rates exists between individual males, but whether or not a given male attacks the nonred dummy more appears to be unrelated to whether he exhibits high or low attack rates. 3. Males showed an increased attack rate after they acquired eggs in their nest, but the tendency for a male to attack the nonred dummy more than the red one appears to be unchanged after he acquires eggs. 4. It is suggested that the nuptial coloration of male threespine sticklebacks be interpreted more generally as an indicator of the male's social and physiological status, emphasizing its intimidating effects as well as its aggression eliciting effects for rival males. This is consistent with the observation that nuptial coloration is capable of increasing avoidance as well as attack in rival males and increasing sexual attractiveness to females.