Sneaking is common in nest-building fish with paternal care, but the role of nest-opening size in protecting against entry by sneaker males has never been tested before. Using the sand goby (Pomatoschistus minutus), a fish with exclusive paternal care, experimental manipulations of nest openings provided no support for the hypothesis that nest openings serve as physical or visual defense or that sneaker males prefer to parasitize nests with wide openings. Female mating preference was also not influenced by nest-opening size. However, female courtship behavior and visibility were important cues for sneaker males. Most sneak entries occurred when the nest holder was occupied with courtship, chasing another sneaker male or nest building. In half the cases of observed sneak entry, the sneaker male fertilized eggs, also when sneaking only occurred before spawning. Sneak entry and its duration were good estimates of stolen paternity, but neither sneak entries nor degree of fertilizations were correlated with filial cannibalistic behavior. Testes size did not explain parasitic spawning success in replicates with genetically determined sneaking. However, all sneaker males without breeding coloration had huge testes and small sperm duct glands, whereas nest-holding males had small testes and large sperm duct glands, and sneaker males with breeding coloration were intermediate.