The endosymbiont Symbiodolus clandestinus, living inside insect cells, has remained undiscovered until now. Scientists at the Max Planck Institute for Chemical Ecology report its discovery in at least six insect orders. Using fluorescence in situ hybridisation, they showed that Symbiodolus is present in all life stages and tissues of infected insects. The symbiont is abundant in reproductive organs of adult females, eggs, and larvae, suggesting transovarial transmission. The interactions with host insects are not yet understood, but Symbiodolus likely impacts insect communities and ecosystems considerably.