Individual differences in coping with potentially dangerous situations are affected by a combination of genetic and environmental factors. How genetic polymorphisms and behavioural variations are related to fitness is unknown. One of the candidate genes affecting a variety of behavioural processes, including impulsivity, anxiety and mood fluctuations in both humans and other vertebrates is the serotonin transporter gene (SERT/SLC6A). The aim of this study was to assess an association between SERT genotypes and novelty seeking, risk-taking behaviours and breeding parameters of great tits (Parus major) in a natural environment. We associated polymorphisms in the promoter exonic regions of the SERT gene with parental risk-taking related behaviour and fitness traits. Our results show that (i) risk-taking behaviour in our great tit population is linked to single nucleotide polymorphisms in the SERT gene exon 3 and exon 8; (ii) the genotype-behaviour associations are consistent at the presence of different stressors; (iii) polymorphisms in exon 8 could be associated with fitness-related traits, such as the start of egg-laying and hatching success. We showed for the first time that genetic variability of SERT plays an important role in shaping individual decision-making that affects fitness consequences in a wild population. However, the results are based on one population and on the polymorphisms that are in one single gene. Therefore, replication studies are needed in order to confirm these preliminary results.