Subtherapeutic doses of chloroquine have been reported to enhance infectivity of drug-resistant Plasmodium species to their vectors. In this investigation, Plasmodium yoelii nigeriensis N67 strain showed enhanced infectivity to mosquitoes when stimulated by chloroquine. Both sensitive and resistant clones were derived from the N67 strain by dilution, showing that this strain is polymorphic for the resistant trait. A resistant subline was derived by selection under drug pressure from a chloroquine-sensitive clone, but neither the sensitive nor the resistant clones or sublines showed enhancement of infectivity in the presence of chloroquine. This suggests that the enhancement of infectivity shown by the N67 strain is a response to chloroquine stimulation shown only by certain of the genotypes within it, and that this response to chloroquine is not a trait causally connected with the genes coding for chloroquine resistance.