Introduced landlocked Rainbow Smelt Osmerus mordax are hypothesized to be a major factor in the decline of Lake Whitefish Coregonus clupeaformis populations in many lakes. We sought to identify the size of Lake Whitefish preyed upon by adult Rainbow Smelt and how the efficiency of Rainbow Smelt predation changes Lake Whitefish ontogeny. In a laboratory setting, we exposed larval Lake Whitefish of increasing sizes to groups of seven Rainbow Smelt (>100 mm) for a 1-h period and observed predation behaviors and efficiencies. In each trial, the group of Rainbow Smelt consumed at least one larval Lake Whitefish, which were up to 45 mm in length (up to 89% of Rainbow Smelt gape width). Predation efficiency, the total number of Lake Whitefish consumed by Rainbow Smelt during trials, was 100% after Lake Whitefish hatching and followed a decreasing sigmoidal response to increasing lengths of Lake Whitefish. The apparent predatory window of Rainbow Smelt on Lake Whitefish is from Lake Whitefish size at hatch (∼12 mm) to approximately 34 mm. Rainbow Smelt generally required multiple attacks to capture a single Lake Whitefish. The capture efficiencies of Rainbow Smelt decreased from 30% as Lake Whitefish length increased and were highly variable within each Lake Whitefish size-group. The overall impact that Rainbow Smelt predation will have on Lake Whitefish populations is dependent on the growth rate of Lake Whitefish, environmental conditions that cause the Lake Whitefish hatching period to coincide with Rainbow Smelt spawning events, and the degree of overlap in habitat use between spawning Rainbow Smelt, nonspawning subadult Rainbow Smelt, and hatching Lake Whitefish.