INTRODUCTION: The serotonergic system has been implicated in several CNS activities, including social behaviour. The serotonin reuptake transporter (SERT) plays a key role in the regulation of serotonergic system functioning. Therefore, rats lacking SERT (SERT- /- ) represent a valuable model to study the consequences of constitutive increases in serotonin concentrations. In adult laboratory rats, two main types of ultrasonic vocalisations (USVs) have been described: the low (22-kHz) and high (50‑kHz) frequency calls. The low, termed an “alarm” vocalization, has been associated with negative social experiences. The high may be detected in appetitive contexts, including social interactions. AIM(S): The goal of the current study was to examine male SERT- /- and SERT+ /+ rats in the social interaction test to investigate genotype differences in social behaviour and communication. METHOD(S): Two unfamiliar rats of matched body weight were placed in the open field arena, and their behaviour was recorded. Durations of the following behaviours were scored: social contact behaviour (including sniffing, anogenital sniffing, social grooming, and mounting/climbing) and following the partner. Additionally, USVs were measured during the social interaction tests. RESULTS: We report that SERT-/- rats spent significantly less time on social contact but demonstrated more of partner following behaviour as compared to SERT+ /+ rats. There were no effects of genotype on the number of 22-kHz and 50-kHz USVs emitted during social interaction. However, serotonin transporter deletion affected the distribution of sound categories in that SERT- /- rats demonstrated a decrease in the percentage of complex calls and an increase in the percentage of trill and step calls. CONCLUSIONS: The current study further supports the role of serotonin in the regulation of social and communicative behaviour. FINANCIAL SUPPORT: This study was supported by the grant ERA‑NET Neuron II JTC 2015 Respond.