The Pacific Marine Environmental Laboratory (PMEL) of the United States' National Oceanic and Atmospheric Administration (NOAA) has been monitoring and archiving sound recordings from the U. S. Navy's underwater hydrophone arrays since 1991. These data contain many marine mammal calls, including those made by blue whales Balaenoptera musculus. The call of the blue whale in the northeast Pacific is often sufficiently loud to be detected on more than one array. Matched filtering techniques can expand detection capabilities to additional arrays, allowing blue whale calls to be localized even when they are far from the arrays. In this manner, PMEL has identified regions where blue whales occur seasonally well offshore of the west coast of the U.S. In order to monitor areas of the world's oceans not covered by fixed hydrophone arrays, PMEL has developed autonomous moored hydrophones that have been used to record acoustic energy from both underwater seismic activity as well as that from whale calls. Each mooring package consists of an anchor, an acoustic release, line, hydrophone and data recorder, and a flotation package. The hydrophones are designed to be moored in the SOFAR channel; the titanium case containing the data recorder can withstand pressure to at least 1000 m below sea level. Currently each hydrophone can store up to 2.8 Gb of data; only the sampling frequency and battery life (up to 8 months, depending on sampling frequency) limit the duration of the experiment. The hydrophones are designed to be deployed as an array of independent instruments whose geometry can be determined by the needs of the experimenter in order to localize acoustic sources of interest. Deployment and recovery of each instrument takes as little as one hour depending upon the platform used. An array of six of these hydrophones was deployed in the NE Pacific in September 1995. One of the goals of this experiment was to compare and "ground truth'' locations of calling blue whales from the U.S. Navy's SOSUS data with those detected on the autonomous hydrophones. During the seven day experiment no blue whales were seen in the area, but two blue whales were detected and then tracked acoustically on the hydrophone array. One of these animals was detected on all three available SOSUS arrays. The locations of the blue whale from the two methods are comparable although the location accuracy is more precise on the local array.