European grayling from a common-gene pool and with a known ancestry were released into headwaters of a river system in Lesja, Oppland, Norway, about a century ago. These grayling dispersed downstream, and as a result there are now several different populations in this river system, all influenced by different environmental factors. Earlier research has found these populations to differ in phenotype, and evidence for genetic structuring of the populations has been found. In this thesis I conduct a common-garden experiment with two temperatures to investigate early life-history traits in different populations of grayling, focusing on development and gene expression (RNA analysis with real-time PCR). For five populations the timing of hatching and eye pigment formation was observed, and for three of these populations I estimated relative mRNA-levels for three target genes; Tropomy, and heat- shock genes HSC70 and HSP70. I found no clear pattern of differences in timing of development rates, and there were no differences in expression of the genes between the populations. I suggest that these results are a consequence of pleiotropy, but find it unlikely for Tropomy and HSC70, due to plasticity shown for these genes. I do find significant differences in expression of Tropomy and HSC70 between temperatures. Individuals incubated at low temperature (6°C) had a higher expression of the Tropomy gene and HSC70 gene, than individuals incubated at high temperature (10°C). This plastic response indicates that the two genes are important for cold tolerance in grayling.