Microsporidia are intracellular parasites of animals and some protists. They are promising agents of biological protection of plants against insect pests in which they cause chronic diseases, ultimately leading to death. In this study, five species of microsporidia of the genus Nosema from different Lepidoptera (the silkworm, cotton bollworm, cinnabar moth, beet webworm, and European corn borer) were tested for their ability to infect larvae of the Colorado potato beetle Leptinotarsa decemlineata (Say). Infection was orally induced by feeding fresh potato leaves treated with microsporidian spores. Only the microsporidium Nosema sp. NspHA from the cotton bollworm Helicoverpa armigera (Hübner) had a negative impact on the growth of the Colorado potato beetle, resulting in delayed development and abnormalities during metamorphosis. Insect mortality was also increased after treatment with spores of Nosema polyvora (Blunck) from the cabbage white Pieris brassicae (Linnaeus). Further studies will include testing low doses of these isolates on beetle larvae and assessing their impact on the viability and fertility of the offspring of infected beetles.