Few studies have been made on the peri-copulatory behaviour and the outcome of multiple mating in secondarily derived cul-de-sac spiders. Here, molecular markers (allozymes) are used to confirm multiple-paternity in wild-collected females of the Silver Stretch spider, Tetragnatha montana, showing that sperm competition is a potentially important driver in the evolution of its mating system. Estimates of P₂ (the relative paternity accrued to second males) were obtained from laboratory-staged double matings. It is shown that paternity can accrue to either or both of the mating males: the long-standing hypothesis that there is a biased paternity advantage to second males as a result of the cul-de-sac spermathecal configuration is, therefore, not supported by this study. Simple measures of male mating performance also do not seem to correlate with paternity, although the sample size of successful egg sacs is small. A model of sexually antagonistic, co-evolutionary manipulation of access to the fertilization set may provide a mechanism for the evolution of the derived cul-de-sac condition in the tetragnathids.