The focus of this chapter is on the role of chemical pesticides in altering the efficacy of plant pathogens, mostly fungi applied as biological control agents. Pathogens are a relatively untapped resource for selective weed control. However, because of their host specificity, pathogens typically control only one out of a complex of weeds in the field, and consequently they must be used in conjunction with chemical herbicides to control the various weeds in a field. Moreover, under agricultural situations, application of pathogens must be in conjunction with chemical herbicides, insecticides, fungicides, and plant growth regulators (e.g., harvest aids) that are intended to manage pests besides the target weed and to facilitate crop production. Hence, it becomes necessary at the outset to understand the types of interactions between chemical pesticides and biocontrol agents in order to prevent failures and to increase the effectiveness of the bioagents. Examples of interactions, including the consequences and the modes of action, between plant pathogens and herbicides have been dealt with in review articles. 1-4 On the other hand, information concerning the interactions of insecticides and fungicides with plant pathogens is scattered through plant pathology and pest control literature. Here we will discuss some specific examples of interactions between chemical and biological weed control agents and how such interactions might facilitate effectiveness of biological control agents.