Abstract A recent experimental study has reported significant changes in the behaviour of the fish Galaxias maculatus due to the presence of the parasite trematode Tylodelphys sp. in its cranial cavity. The underlying mechanisms of these behavioural changes remain unknown. This study aims to elucidate potential histological alterations caused by the trematode Tylodelphys sp. in G. maculatus hosts.Specifically, cranial tissues from parasitized and non-parasitized fish were compared to assess possible structural changes or lesions associated with the presence of the parasites.It was found that Tylodelphys sp. does not induce visible changes in the brain, meningeal tissue, or cranial bones of the fish. Tylodelphys sp. primary resides in the posterior region ofthe cranial cavity, within the cerebrospinal fluid surrounding the brain near the hypothalamus, basal nuclei, and other structures related to vision and locomotion. Therefore, Tylodelphys sp. appears to influence the behaviour of its fish host without causing direct damage to the brain tissue, possibly through increased cerebrospinal fluid pressure or chemical interactions with the host’s brain, mechanisms that should be further investigated in future studies.