A sample survey of breeding numbers of Manx Shearwaters Puffinus puffinus was carried out on Rum in 2001, based on 658 circular quadrats randomly positioned throughout the colony. All burrows in the quadrats were checked for occupancy during the late incubation period by playing a tape of a male shearwater call at burrow entrances and recording any response. In order to account for non-responding occupants, the estimate of the total number of responding burrows was multiplied by a correction factor of 2.16, derived from a calibration study carried out in 2003. A total population estimate of 76,310 occupied burrows (61,160 – 95,740, 95% CL) resulted. Burrows were also checked visually for signs of occupancy and this suggested a higher total of 119,950 occupied burrows (106,730 – 133,550, 95% CL). Further study of the response rate at this colony is advisable in order to understand the discrepancy between these findings. The estimate based on visual signs of occupancy is comparable with earlier studies, and little evidence exists to indicate that the colony is decreasing, as was suggested in the 1990s.