Host–parasite coevolution can lead to a variety of outcomes, but whereas experimental studies on clonal populations have taken prominence over the last years, experimental studies on obligately out‐crossing organisms are virtually absent so far. Therefore, we set up a coevolution experiment using four genetically distinct lines of Tribolium castaneum and its natural obligately killing microsporidian parasite, Nosema whitei. After 13 generations of experimental coevolution, we employed a time‐shift experiment infecting hosts from the current generation with parasites from nine different time points in coevolutionary history. Although initially parasite‐induced mortality showed synchronized fluctuations across lines, a general decrease over time was observed, potentially reflecting evolution towards optimal levels of virulence or a failure to adapt to coevolving sexual hosts.