Phenotypic plasticity allows organisms with identical genotypes to develop different phenotypes in response to different environmental cues. Phenotypic plasticity is a trait in itself, and can be either adaptive or non-adaptive. In this study I test for a potential salinity-induced plastic effect on the development of lateral bony plates in a fresh water population of the threespine stickleback (Gasterosteus aculeatus), a fish known for having plastic responses to other environmental cues. In order to do this, I bred male and female sticklebacks, partitioned each family of offspring into two groups, and reared each group in two different environments (salt and fresh water). This kind of experiment is called a split-clutch design and makes it easier to differentiate genetic and plastically induced differences. 16 families survived until the experiment s end, and when the fish had reached the size where all plates were fully developed, plate number was counted for all fish. These data were analyzed using a general linear mixed model to see if plate number was affected by the different environments. The results showed that there is a plastic effect on plate number induced by increased salinity: sticklebacks reared in fresh water developed more plates than those in salt water. This was somewhat contrary to my hypothesis, as wild sticklebacks consistently show a considerably lower number of plates in fresh water populations than in salt water populations. This along with the fact that the observed difference in plate number was relatively small, indicates non-adaptive rather than adaptive plasticity. However further research is needed in order to confirm or deny this.