The longevity and productivity of Taenia taeniaeformis were studied in experimentally infected cats. Nineteen of 20 cats became infected after they were given 8 to 12 strobilocerci. In 8 cats, mean prepatent period was 47.1 days +/- 5.9 SEM (34 to 80 days). Patent periods in the infected cats were 7 to 34 months. In 11 cats in which the infections were allowed to terminate naturally (spontaneous recovery), mean patient period was 17.4 months +/- 2.3. Two of these cats were then given a 2nd dose of strobilocerci and both became reinfected. The mean daily proglottid output in 6 cats followed throughout the patent period was 4.3 +/- 0.5. Destrobilization occurred spontaneously and sporadically throughout the infection, but did not always increase at the time of natural termination. Usually, proglottid productivity began to decrease after the 1st year of infection. Shed proglottids contained 0 to 12,180 eggs, with a mean of 1,606 +/- 402, but more than 60% of the segments contained 500 or less. Taenia taeniaeformis appeared to be as persistent as other taeniids, such as T ovis and T hydatigena in the dog, but it was far less prolific as an egg producer.