The human contribution to ambient noise in the ocean has increased over thepast 50 years, and is dominated by low-frequency (LF) sound (frequencies<1000 Hz) from shipping, oil and gas development, defence-related andresearch activities. Mysticete whales, including six endangered species,may be at risk from this noise pollution because all species produce andprobably perceive low-frequency sound. We conducted a manipulative fieldexperiment to test the effects of loud, LF noise on foraging fin blue(B. musculus) and (Balaenoptera physalus) whales off San Nicolas Island,California. Naive observers used a combination of attached trackingdevices, ship-based surveys, aerial surveys, photo-identification andpassive monitoring of vocal behaviour to examine the behaviour anddistribution of whales when a loud LF source (US Navy SURTASS LFA) wasand was not transmitting. During transmission, 12-30% of the estimatedreceived levels of LFA of whales in the study area exceeded140 dB re 1 μPa. However, whales continued to be seen foraging in theregion. Overall, whale encounter rates and diving behaviour appeared tobe more strongly linked to changes in prey abundance associated withoceanographic parameters than to LF sound transmissions. In some cases,whale vocal behaviour was significantly different between experimentaland non-experimental periods. However, these differences were notconsistent and did not appear to be related to LF sound transmissions. Atthe spatial and temporal scales examined, we found no obvious responses ofwhales to a loud, anthropogenic, LF sound. We suggest that the cumulativeeffects of anthropogenic LF noise over larger temporal and spatial scalesthan examined here may be a more important consideration for managementagencies.