The Heard Island Feasibility Test source transmitted a hum at 209-220 dB re: 1 microPa at 175-m depth, centered on 57 Hz with a maximum bandwidth of 30 Hz for 1 h of every 3. Experienced marine mammal observers conducted line-transect surveys and monitored marine mammal behavior visually and acoustically in a 70 x 70 km square centered on the transmission site. Thirty-nine groups of cetaceans and 19 of pinnipeds were sighted from both vessels before the start of transmissions. Thirty-nine groups of cetaceans and 23 of pinnipeds were sighted during transmissions. Blue (Balaenoptera musculus), fin (B. physalus), and sperm (Physeter macrocephalus) whales were sighted during the base line period; blue, sperm, and possibly sei (B. borealis) whales were sighted during the transmission period. More schools of hourglass dolphins (Lagenorhynchus cruciger) were sighted during transmissions, but fewer groups of pilot whales (Globicephala melas), southern bottlenose whales (Hyperoodon planifrons), and minke whales (B. acutorostrata). The density of all cetaceans was 0.0157 groups/km2 before the transmissions and 0.0166 groups/km2 during. Antarctic fur seals (Arctocephalus gazella) and southern elephant seals (Mirounga leonina) were seen, but not in sufficient numbers to estimate abundance. One blue whale tracked before, during and after a transmission changed respiration and reorientation rates, but did not avoid the source detectably. Sperm whales and pilot whales were heard in 23% of 1181 min of baseline acoustic surveys; but in none of 1939 min during the transmission period. Both species were heard within 48 h after the end of the test.