Oxitec has made significant progress toward securing permission for the trial: Late last year, it won approval from several federal agencies to import mosquito eggs and build a lab for inspection. In the lab, scientists plan to inject the eggs with synthetic DNA, rear the mosquitoes and release them in Key Haven, once the field trial is permitted. The lab is in the Marathon, Fla., office of the mosquito control district, which is working with Oxitec on the project.

Oxitec seeks to drastically reduce the population of the dangerous and hard-to-kill Aedes aegypti here by freeing male mosquitoes with a specially made gene designed to kill their offspring after they mate in the wild. This, in turn, could blunt the spread of dengue and chikungunya, viral diseases that have no cure and are spreading quickly around the world.

For Oxitec, the Key Haven trial would be just one of several.

More than 70 million Oxitec mosquitoes have been released in field trials in the Cayman Islands, Malaysia, Brazil and, most recently, Panama, all of which have struggled with dengue. Regulatory agencies in those countries approved the release of the mosquitoes, and last year Oxitec received approval from Brazil to release its mosquitoes commercially.

Trying to unleash a better weapon to curb dengue, which hit Key West in 2009 and 2010, and chikungunya is a smart preventive, said Michael S. Doyle, the executive director of the mosquito control district, which invited Oxitec to conduct the trial.

Pesky and potentially dangerous, the virus-carrying aegypti mosquitoes are difficult to kill and snack on humans almost exclusively. The Keys mosquito agency, regarded as one of the best in the country, can kill only 50 percent. Aegypti make up 1 percent of the Keys mosquito population but require 10 percent of the budget, Mr. Doyle said.

The aegypti prefer urban settings. They love backyards (not marshes), bite during the day and easily breed in tiny spaces (soda bottle caps, for example). Insecticides, which can harm other organisms, often miss them. Only the females bite, which is how the diseases are transmitted.