There is a paucity of information pertaining to roof rat resistance in the state of California, particularly from the roof rats (Rattus rattus). Roof rats cause extensive damage in both urban and agricultural sites. Several tree crops, including citrus and nuts, experience costly losses from rat damage to fruits and nuts, and from girdling of trunks and branches. Rats are also a significant public health threat in urban areas and pose food safety risks in crops from contamination with their urine and feces. The first-generation anticoagulants (FGARs) chlorophacinone and diphacinone are labeled to protect crops from roof rat damage and are replacing the now-banned second generation anticoagulant rodenticides (SGARs) for commensal uses. The risk of resistance to these compounds needs to be assessed in the context of strong selective pressure. The main objective of this research is to investigate the prevalence of rodenticide resistance in Rattus species in urban and agricultural areas in the state of California.