Compared to female mate choice, relatively little is known about male mate choice and the selective pressures that shape it. My thesis focuses on the influence of available and inexpensive social information on male mating behaviour, including mate choice. To gain insights into the influence of the social environment on male mating behaviour, I carried out seven experimental studies using a promiscuous fish, the Trinidadian guppy (Poecilia reticulata), as a model species. I used guppies from the low-predation, Upper Aripo River in Trinidad. In dichotomous choice tests, females in this population demonstrated a strong mating preference for large males and a weaker, but significant, preference for males with greater amounts of colour ornamentation. In the absence of sexual rivals, male guppies exhibited an overall preference for large females, but with consistent and repeatable inter-individual variation in both mating effort and mate choice. When conspecific males were present, male guppies paid attention to their social environment and, when given the opportunity, copied the mate choice of a nearby sexual rival. In theory, males should adjust their mating behaviour so as to minimize the potential costs of being copied. As expected, males decreased their preference for an initially preferred female in the presence of a larger, more competitive sexual rival. They also decreased their overall mating effort and used less conspicuous, but also less successful, sneak mating attempts in the presence of an audience of one or two rivals. Since the social environment can influence male mating behaviour and thus mating success, males should strategically associate with sexual rivals in a manner that will enhance their own reproductive success. Here, males preferred to socially associate with conspecific males who they had seen actively performing courtship displays towards a female compared to those who did not. Males who perform courtship displays may be seen as desirable social partners, as they are attractive to females and(or) may be providing social information regarding the perceived quality and(or) sexual receptivity of nearby females. Overall, my thesis highlights the important influence of the social environment on the evolution of male mating behaviour and social partner choice.