Listed below are some of the more common biological control agents used in mosquito control.

Bacillis thuringiensis israelensis (Bti): Bti is a bacteria used to control mosquitoes.
(label-msds) Bti can be purchased locally by homeowners and does an excellent job controlling mosquitoes in the larval stage.

Bacillis sphaericus (Bs): Is another bacteria used to control mosquito larvae and under the right conditions, can remain effective for a couple weeks
(label-msds)

Mosquitofish: Visit the mosquitofish web page which includes discussions on the use of mosquitofish along with taxonomy, video and more. HTML

Purple Martins: Visit The Purple Martin Conservation Association web site. The following excerpt was taken from their website. HTML

"Martins, like all swallows, are aerial insectivores. They eat only flying insects, which they catch in flight. Their diet is diverse, including dragonflies, damselflies, flies, midges, mayflies, stinkbugs, leafhoppers, Japanese beetles, June bugs, butterflies, moths, grasshoppers, cicadas, bees, wasps, flying ants, and ballooning spiders. Martins are not, however, prodigious consumers of mosquitoes as is so often claimed by companies that manufacture martin housing. An intensive 3-year diet study conducted at PMCA headquarters in Edinboro, PA, failed to find a single mosquito among the 350 diet samples collected from parent martins bringing beakfuls of insects to their young. The samples were collected from martins during all hours of the day, all season long, and in numerous habitats, including mosquito-infested ones. Purple Martins and freshwater mosquitoes rarely ever cross paths. Martins are daytime feeders, and feed high in the sky; mosquitoes, on the other hand, stay low in damp places during daylight hours, or only come out at night."

Bats: The following excerpt was taken from the Bat Conservation International web site. The article, Bats, Man-Made Roosts, and Mosquito Control, discussed the use of bats for mosquito control.  HTML

"Bats are primary predators of vast numbers of insects that fly at night, and some species consume large numbers of mosquitoes. However, mosquito control is a complex problem that rarely can be solved by a single approach, be it bat houses or pesticides."