Current theory predicts that males should guard prereproductive females if they will thereby achieve sperm priority and that the propensity of males to guard prereproductive females, and aggression exhibited by both sexes at this time, should be related to their density and sex ratio. Low female density should select for males to guard prereproductive females, but a female-biased sex ratio should weaken any propensity for males to guard them. Low density and a female-biased sex ratio should favor a low level of aggression by both males and females. We tested these predictions with a low-density, female-biased population of the crab spider Misumena vatia. During experimental introductions only 10% (n = 40) of the males guarded penultimate-stage females through their final molt to copulate with them, even though penultimate females seldom responded aggressively to males in these trials (10%). Nearly all males mated soon after they found newly molted adult virgin females (94%: n = 31). Males guarded only 8% (n = 39) of the penultimate females censused in the field, but each male mated with the female it guarded as soon as that female molted. Only 22% (n = 23) of trials with two males and a penultimate female resulted in more than momentary physical contact between the males, and none resulted in injury. Thus, both predictions were based on the sex ratio, but only one of the two associated with population density, were confirmed. These results suggest that sex ratio is more important than density in dictating precopulatory guarding and aggression. The strongly female-biased sex ratio of Misumena may explain why males guard pre-reproductive females less frequently and exhibit less aggression than males in a population of the closely-related Misumenoides formosipes.