Abstract: Insecticidally-active bacterial cultures and methods of maintaining the virulence of the bacterial cultures. Several species of the bacteria are harvested from nematodes of the families Steinernematidae and Heterorhabditidae maintained in suitable insect hosts and then cultured as separate, isolated cultures under conditions suitable to the reproduction of the particular bacterial specie(s). Bacteria are harvested for use as an insecticide during the exponential phase of population growth so that the bacteria are highly motile, optionally mixed with other populations of bacteria harvested and cultured from suitable nematode hosts, diluted in water and applied to the insects to be controlled by, for instance, spraying or, in the case of such insects as ants and termites, by pouring the dilute, harvested bacteria onto the insect mound. To maintain the virulence of the isolated bacterial cultures, the bacteria are periodically re-isolated from the nematode, infected in the insect host, and then re-cultured.