Summary--Twenty-two outpatients who sought behavioral treatment for a severe animal phobia were questioned via structured interview to obtain information on the mode of onset and course of development of their fear. Information was also obtained regarding feared consequences and distressing stimulus characteristics of their feared animal. In each case, the phobia began in early childhood and remained stable or worsened with age. Mode of onset was unclassifiable in 15 cases (68~) because of patient inability to remember precipitants. Of the remaining 7 patients, 5 ascribed their fear to a frightening encounter with the animal ('conditioning'); indirect ('instructional' and 'vicarious') causes were cited in the remaining cases. Virtually all patients feared panic and its consequences following an unavoidable encounter with the phobic animal, whereas less than half reported fear that the animal would attack. Stimulus features, especially movement, were found to be salient fear-eliciting cues.