Technical Abstract: Transmission of ovine progressive pneumonia virus (OPPV), a lentivirus of sheep, occurs through both maternal and non-maternal means. Currently, the contribution of each route to the overall flock OPPV prevalence is poorly understood since previous serological epidemiologic studies lacked the ability to accurately track routes of transmission within an infected flock. In this study, the amount of maternal OPP transmission was assessed in a naturally infected ewe flock by applying molecular analyses to proviral sequences derived from peripheral blood leukocytes of OPP positive dam-daughter pairs (N=40). Both proviral envelope (env) and long terminal repeat (ltr) sequences, separately and combined, were utilized in the following two sequence analysis methods: phylogenetic analysis and pairwise distance calculations. True maternal transmission events were defined as agreement in 2 out of the 2 sequence analysis methods. Using this criterion, proviral env sequences resulted in a 14.3% maternal transmission frequency, and proviral ltr sequences resulted in a 10% maternal transmission frequency. Both proportions of maternal transmission varied significantly from equality (P <0.0001). This indicates that the remaining 85.7-90% of daughters are infected via non-maternal transmission. This is also the first study to calculate the OPP proviral rate of change for env and ltr genes. Accurately defining the routes of OPPV transmission provides critical epidemiological data supporting management intended to reduce flock transmission and viral dose.