A telemetry pill was used to monitor core body temperature of penned sea lions. The pill emitted radio frequency (88-108 MHz) pulses at a rate proportional to body temperature. The emitted pulses were received as clicks on a common transistor radio. The pill, which was 1.1 cm diameter and 2.5 cm long, was inserted in a fish which was fed to the sea lion. Range exceeded 5 meters when the sea lion was in the air or in small fresh water pools, whereas immersion in sea water blocked transmission. The 2.5 cm long pills stayed in the animals for several days to weeks. Increasing pill length to 5 cm reduced the time to approximately 2 days. Mean core temperature (4 animals, 97 observations) was 38.1 C, with a standard deviation of 0.4 C. The pill has detected febrile response to disease, and has been used to assist in monitoring postsurgical recovery.