The white–spotted colour morph of the marine isopod Idotea baltica appears cryptic on the brown alga Fucus vesiculosus with its white–coloured epizoites Electra crustulenta and Balanus improvisus. This study shows that the crypsis of this coloration is achieved through disruptive coloration rather than through background matching. Crypsis through background matching requires that the sizes and the shapes of the pattern elements should closely resemble those of the visual background. Comparisons between the white spots of the isopods and those of their natural background contradicted this prediction. Disruptive coloration, which aims to obscure the true form of the animal by partly blending with the background and distracting the attention of the viewer from the contour of the animal to unessential patterns, presupposes more marginal elements than expected by the pattern element distribution in the background, and also highly variable and complex elements. Comparison between the observed spot distribution and simulated individuals with randomly distributed spots showed that the spots in these isopods do indeed touch the body outline more often than expected. Furthermore, the spots were highly variable and complex.