Hypothesis: Parasite infection of juvenile stickleback increases during their early ontogenesis owing to transmission from adults and other juveniles, as well as changes in diet. Organisms: Threespine stickleback (Gasterosteus aculeatus) aged one week to two months, their ecto- and endoparasites, and their stomach contents. Times and places: July to September 2012 and 2015; two locations in the Kandalaksha Bay of the White Sea – Seldyanaya Inlet with dense seagrass beds and an unnamed lagoon in Sukhaya Salma Inlet. Methods: Quantitative sampling of stickleback at 10 day intervals, and quantitative analysis of their parasites and stomach contents. Results: As sticklebacks grew, their parasite load increased. We identified three size groups of stickleback that differ significantly in their parasite species composition and infection indices: hatchlings 7.0–8.5 mm long were infected with three parasite species (prevalence 43%); juveniles 9–11 mm harboured four or five species (100%); and juveniles 12–30 mm were i