Maternally inherited bacterial endosymbionts such as Wolbachia can potentially have a major impact on the reproduction of their arthropod hosts. Most previous studies have focused on the effects on insects, but recent evidence demonstrates that the endosymbionts Wolbachia, Rickettsia, and Spiroplasma are also common in spiders. Such infections potentially explain observed characteristics of reproduction in this group such as skewed sex ratios or reported cases of parthenogenesis. Here we test spiders and a range of other arachnids for infection with another, more recently described maternally acquired endosymbiont, Cardinium. We present data from a survey of spiders and other arachnids collected in the field and obtained from museum collections. Infections with Cardinium are found to be very widespread, perhaps more so than in other arthropod groups. The consequences of this and directions for future research on endosymbiont-arachnid interactions are discussed.