Infanticide by males in species with high maternal investment may be the most extreme example of a reproductive conflict of interests between males and females. The subsocial spider Stegodyphus lineatus is semelparous, and the offspring eat their mother. Male and female maturation overlap, such that late maturing males encounter females that are already guarding the single clutch. A late male may try to remove the existing egg sac, thereby forcing the female to replace the clutch and use his sperm to fertilize at least some of the eggs. Males were responsible for 33% of all the egg losses in a population of 278 females with eggs. By losing her clutch, the female loses time equivalent to the age of the lost clutch plus the time it takes to replace it. The earlier in the season a female lost a clutch and the younger the eggs were, the longer was the interval between clutch loss and clutch replacement. Females that lost their egg sacs by infanticide replaced them after a significantly longer time than females that lost their egg sacs from other causes. The time loss of 18.5 days on average corresponds to a 23% reduction in female survival probability. In addition to reduced survival, females suffered a fecundity cost: replacement clutches were smaller than the original clutches and the reduction in egg number was greater with increasing intervals between the first and second clutch. Females defended their egg sacs aggressively against males. In field tests, the outcome of fights between males and females was determined by the relative size difference of the contestants. The mating system of S. lineatus provides an example of a male mating strategy that evolved despite being very costly for the females.