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The present article tried to establish dark/light preference in five different species of teleosts. We proposed, using the data obtained with this method in zebrafishes (Danio rerio), Cardinal-tetras (Paracheirodon axelrodi), lambaris (Astyanax altiparanae), Nile tilapias (Oreochromis niloticus), guppies (Poecilia reticulata) and banded-knife fishes (Gymnotus carapo), that preference for dark environments is a reliable and low-cost index of anxiety/fear in those species. A scototactic pattern of exploration was found in all species, and the pattern of locomotion in the white environment suggests its aversiveness for those species, with the exception of G. carapo and O. niloticus. A comparative analysis uncovered species differences in approach-avoidance dimensions of the task. The data are discussed in terms of the behavioral ecology of the animals and prey-predator relationships, suggesting a link with predator defense strategies in teleost.The dark/light preference model is already established as an "ethoexperimental" anxiety model in rodents (cf. Bourin & Hascöet, 2003). It is based on the natural aversive quality of brightly-lit environments for mice, shaping -352 -a conflict situation in which the animal must deal with its natural tendency to explore in face of the aversiveness of the environment. The rodent dark/light preference model is an exploration model, in the sense that it measures locomotor activity in both environments as an index of anxiety (Green & Hodges 1991; Prut & Belzung 2001; Belzung & Griebel 2003; Hascöet, Bourin, & Dhonnchadha, 2001); there are other, non-locomotor, models of anxiety (eg., inhibitory avoidance), but those are not of concern for the objectives in this article. Locomotor models of anxiety use exploratory behavior (defined as "a speciesspecific behaviour pattern concerned with the gathering of information" concerning the environment: O' Keefe & Nadel, 1978, p. 242) as an index of anxiety or anxiety-like states, relating it to foraging behavior or to appraisal of novel environmental stimuli (Belzung & Griebel 2001;File, 2001). The main rationale is that exploratory behavior would correlate with neophobia, a tendency to avoid new environments (Misslin & Cigrang 1986), forming a mixed pattern of behavior that consists in gradual approaching and exploration of the new environment associated with "scanning" and "risk-assessment" behaviors.Ethoexperimental models use variables that are akin to the concept of "antipredator apprehension" from behavioral ecology (risk assessment, defensive distance, predatory imminence continuum, risk associated suppression of competing motivational systems; Kavaliers & Choleris, 2001). Apprehension is considered to reflect a motivational state, and is defined as "any reduction in attention to other activities (e. g., foraging, mate seeking) as a result of increasing the allocation of attention to detecting and/or responding to potential predators" (Kavaliers & Choleris, 2001, p. 579). Exploratory apprehensive behavior (denoting the pattern of ex...