Divergent lateral plate phenotypes in stickleback represent one of only a few cases known where a single gene underlies the phenotype under divergent selection between different habitats. However, the selection pressures leading to the repeated loss of lateral plates in freshwater are still not well understood. By genotyping 838 individuals from nine independently colonized lakes and one marine population in Iceland, we found i) that only in some lakes are phenotypes associated with the expected genotype and ii) that the independent repeated occurrence of a rarely described plate phenotype is expressed in the absence of an allele that is usually associated with this phenotype. This suggests that either other genes such as modifiers might be under divergent selection between lakes or that lateral plate expression in these populations is restricted due to environmental constraints.