Although audiograms are available for many odontocete species (toothed whales), no empirically measured audiograms exist for any species of baleen whale. This study used a behavioral response paradigm and exposed humpback whales (Megaptera novaeangliae) to frequency-modulated signals as way of determining their hearing sensitivity in ocean noise. The environment was relatively quiet, with singing whales and snapping shrimp being the most common noise sources. Experiments tested three initial frequencies predicted to be audible (0.25, 1, and 4 kHz), and three additional frequencies, hypothesized to be less audible (0.08, 16, and 22 kHz). As a group of whales swam toward the source, they were exposed to low-level one-third octave upsweeps. If the group significantly changed behavior, the received level, and signal-to-noise ratio (SNR), of the upsweeps were estimated at the position of the behavioral change. For all frequencies, results from the sample population were used to make inferences about frequency-dependent hearing in this species. MRLs at frequencies predicted to be audible to humpbacks were indicative of masked hearing levels and similar to critical ratios in odontocetes. Response levels at frequencies assumed to be at the edges of their hearing range were likely to be unmasked, and therefore indicative of hearing response thresholds.