Technical Abstract: Phomopsis seed decay (PSD) of soybean is the major cause of poor seed quality in most soybean-growing countries. This disease is primarily caused by the fungus Phomopsis longicolla. Few soybean cultivars currently available for planting in the U.S. have resistance to PSD. To identify soybean lines with resistance to PSD, seed of 208 representative maturity group V soybean plant introductions (PI), obtained from the USDA Soybean Germplasm Collection in 2006, were plated and assayed for the percentage of Phomopsis seed infection. Based on this data, 122 PIs without Phomopsis seed infection were selected and field-screened under natural infection in Stoneville, MS in 2007. Based on the results of assays of naturally infected seed from 2006 and 2007, 14 PIs were selected for further evaluation with inoculated and non-inoculated treatments in 2008 and 2009. PI 424324B was identified as the most PSD-resistant line. It had zero Phomopsis infection in the seed assays in 2006, 2007, 2008, respectively. In 2009, frequent rainfall during seed maturation led to high levels of seed infection by Phomopsis (up to 80%) and other fungal pathogens for most soybean lines tested in Stoneville, Mississippi. However, PI 424324B had only 1% and 2% seed infected by Phomopsis from the non-inoculated and inoculated treatments, respectively. In addition, PI 458130 was also resistant to PSD, with no seed infection from the naturally-infected trials in 2006 and 2007 and less than 3% Phomopsis seed infection in the 2008 and 2009 inoculated trails. These resistant sources will be used to develop cultivars resistant to PSD.