This research was carried out in order to show that certain substances of algal origin are toxic to animals, particularly to fish, and to determine if a relationship exists between toxicity and the latitude where the algae were collected. Twenty‐nine species of algae were collected in different localities off the Pacific coast of Mexico. The samples were frozen shortly after collection and kept frozen until used. Crude ethanolic extracts of the algae were prepared and tested for ichthyotoxicity. The extracts were classified as nontoxic (NT), weakly toxic (WT) and highly toxic (with lethal effects) (T). The reactions exhibited by the fish to the presence of the algal extracts in their environment ranged from initial adaptative behaviour to death. Only four species (14%) out of the 29 algal species tested were highly toxic, 13 (45%) weakly toxic and 12 (41%) non‐toxic. The high percentage of algal extracts that showed some degree of toxicity suggests that algae might have some kind of defence mechanism that could be noxious, thus being inedible by fish. The highest toxicity values were found among tropical algae. These results suggest that a relationship could exist between toxicity and the latitude of the collecting locality.