Marbled Murrelets Brachyramphus marmoratus feed entirely at sea. However, little is known of their distribution within the marine environment. Such knowledge would allow agencies to identify critical foraging and loafing areas that could be protected, especially relative to nesting habitats. The purpose of the present study was to examine the at-sea locations of radio-marked Marbled Murrelets during the breeding season relative to: (1) the shoreline and old-growth forest, (2) the maximum extent of alongshore travel during the breeding season, and (3) home range size on the ocean. For each of these measures, we also stratified the analyses by sex and nesting status (nested after capture or did not nest after capture). Using aircraft telemetry, we followed 102 Marbled Murrelets in the coastal waters of northern California between 2001 and 2003. Over the three years, murrelets were detected 1.4 ± 0.1 km (n = 93; mean ± standard error) from shore regardless of sex or nesting status. Murrelets traveled a maximum of 99.1 ± 9.5 km (n = 94) alongshore (north–south direction). Male murrelets tended to travel a greater distance than did females, and male murrelets that did not nest traveled significantly more alongshore than did male murrelets that nested after capture. Average home range size (minimum convex polygon) was 505 ± 75 km2 (n = 94) and was larger for males than females. Home range size was larger for non-nesting murrelets than for nesting murrelets. When the data were stratified by sex and nesting status, home range size was larger for non-nesting males than for nesting males. However, home range size was similar for non-nesting and nesting female murrelets. Our data suggest that Marbled Murrelets in northern California occupied nearshore waters, and traveled less than 50 km away from the mouth of Redwood Creek, CA, the prominent watershed where most nesting occurred. However, the data also suggest that non-nesting males can make long-distance movements, perhaps in search of mates or nesting habitat.