The authors describe a case of acute intoxication by bracken fern ( Pteridium aquilinum) in a cattle herd raised in the city of Reserva, Parana State. A total of 25 animals died due to intoxication caused by the ingestion of large amounts of the toxic plant after the occurrence of a frost period in the winter month of July, 2000. Beside the herd clinical case, the authors present some considerations about the spreading of bracken fern in Parana, with particular interest in the environmental conditions (including weather). We suspect that these conditions can facilitate the spread of the plant during the cold months of the year. More plant available means a favorable situation for the bracken fern animal consumption, then causing losses to the cattle industry.