This paper presents observations made with detectors aboard Imp 8 of a unique series of charged particle bursts that occurred on February 16, 1974, while the spacecraft was traversing the dawn magnetosheath region. This event was unlike any other observation of Imp 7 and 8 in 5 1/2 years of operation in orbit, and, to our knowledge, no other observation of this type has been reported. The measurements are most directly interpreted as an intense, 6¿103 (cm2 s sr MeV/nucleon)-1, highly collimated (≲5¿ width) beam flowing in the antisolar direction composed of medium (carbon, nitrogen, or oxygen) nuclei. Both the degree of collimation in arrival direction and the composition are unique to this event and have never been observed in the hundreds of magnetospheric bursts or solar and interplanetary events present at these distances (20--35 RE) from earth. We present this event as a possibly important signature of terrestrial O+ ions escaping from the magnetosphere. Since it is a unique observation, we have examined the instrument performance very carefully and have concluded that it is valid.