Carbon nanotubes (CNTs) can be produced with a wide range of physicochemical properties, including diameter, length and surface chemistry. In addition, CNT production is increasing due to their incorporation into a growing number of applications, including in composite materials and environmental sciences (e.g. remediation). This is a cause of concern, as their release to the environment may increase rapidly in combination with a limited understanding of their potential for exposure and subsequent effects. In the present study, the aim was to investigate if 3 differently functionalised multi-walled CNTs (MWCNTs; COOH, OH and NH2) and one non-functionalised MWCNT (pristine; MWCNT-P) could induce cytotoxic and genotoxic damage in vitro in liver cells of Poeciliopsis lucida (PLHC-1). CNTs with similar physical properties with respect to average diameter and length were selected so that any potential damage could be attributed solely to differences in the type of functionalisation. Cytotoxicity and genotoxicity were assessed by the MTT assay and agarose gel electrophoresis respectively. A relative measure of DNA double strand breaks (DSBs) was determined by image data analysis to quantify the DNA fraction, of total DNA, that migrated into the gel (DNA-FTM), as well as the median molecular length (MML) of the DNA fragments that left the well during gel electrophoresis.. The results demonstrated that the MWCNTs induced some cytotoxicity at relatively high concentrations (5-100µg/mL), with MWCNT-COOH and MWCNT-NH2 being the most and least cytotoxic, respectively. A significant increase in DSB frequency was observed inn cells treated with a high concentration (64 µg/mL) of MWCNT-P, relative to the low exposure (4 µg/mL). In contrast, no significant differences in DSB frequency were observed in cells treated with MWCNT-COOH, - OH and NH2.However, investigation of MMLs of the damaged DNA indicated that MWCNT-OH and MWCNT-NH2 were both genotoxic; both showed a significant decrease in MML when comparing to the high (64 µg/mL) and low (4 µg/mL) exposures respectively. This study indicates that the risk of cytotoxic effects from these MWCNTs to PLHC-1 cells is relatively small. The study also suggests that some genotoxic effects may occur in PLHC-1 cells at low and high exposure concentrations of MWCNTs. The CNT surface chemistry appears to play a role in the degree of cytotoxicity and genotoxicity observed.