Celera Genomics Corp., the Rockville company that roiled the scientific world when it announced a high-stakes private venture to unravel the human genetic code, said yesterday that it had passed a critical milestone in a related endeavor to map the genes of the fruit fly.

The company said it had completed the physical process of reading, or "sequencing," the fruit fly's entire genetic code. Several months of work remain to assemble the resulting data and turn them into a scientific paper.

Celera has built by far the largest single gene-mapping laboratory in the world. Its announcement yesterday suggests how well that lab works: Using a few dozen employees working roughly from Memorial Day to Labor Day, the lab accomplished a series of steps that once would have taken many years and many thousands of scientists to pull off.

"This took laboratory breakthroughs at every level," said J. Craig Venter, Celera's president and chief scientific officer. "What it shows is that the new technology is just spectacular."

The announcement is important to Celera's investors largely as a sign that the company may well be capable of mapping all human genes, a potentially lucrative undertaking. However, the fruit fly project is vital to science in its own right. However annoying it may be to find them on your bananas, fruit flies are essential research subjects.

For 90 years the tiny fly, Drosophila melanogaster, has been the model organism for the study of genetics and developmental biology. More than 100,000 scientific papers have been published on the fruit fly, and some 5,000 researchers around the world work full time studying it.

Fruit flies have complex body plans, including a central nervous system, and they exhibit elaborate behavior. More than half their genes are thought to bear close resemblance to human genes. Much current knowledge of human genetics, including the genetics of inherited diseases, derived initially from studying the fruit fly.

Fruit-fly research has led to the award of three Nobel Prizes in medicine. The most recent was in 1995, when three scientists won for a series of studies on how fly embryos develop--research that led directly to dramatic discoveries about how human babies develop and what can go wrong in the process to produce birth defects.

Venter is a pioneer in developing rapid methods of gene research. Celera is a subsidiary of PE Corp., a Norwalk, Conn. maker of scientific instruments, including an extremely fast new gene analyzer that uses a robot arm to eliminate many tedious steps once performed by lab technicians. That machine is at the heart of Celera's efforts. Venter has installed nearly 300 of them, three times as many as any other laboratory.

Venter is competing with an international consortium of academic scientists, funded by billions from the federal government and the world's largest charity, to be the first to compile a complete map of human genes. His claim a year ago that he would do this shocked the academic scientists, who had assumed they could pursue their research at a leisurely pace.

By the time the dust settled, the academic researchers had drastically accelerated their timetable to keep from being rendered irrelevant by Venter. They are now promising a "rough draft" of the human gene map, one good enough to jump-start medical discoveries, by next spring, with a definitive map due in 2003. Venter is promising a virtually complete map by 2001, though the buzz around Celera is that it may happen sooner. Venter has been signing up big drug companies that are willing to pay millions a year for access to his data and research tools.

Despite the scale of Venter's operation, doubt has lingered in the scientific community about his ability to pull off the feat. Yesterday's announcement may assuage some of that doubt, by showing that Venter's small staff of scientists can physically manage the task of chopping a complex organism's genes into pieces tiny enough to be read by his enormous battery of gene analyzers. Indeed, after the last fruit-fly genes were read on Labor Day, Venter's technicians began pumping human genetic material into the pipeline on Tuesday.

The steps accomplished so far on the fruit fly, significant as they are, are not necessarily the hardest. Venter and his people now have millions of tiny pieces of data representing small bits of the fly's genetic code. They must use supercomputers to assemble all those bits into a whole relatively free of error. It's the completed map, the fruit fly's entire "genome," that scientists want.

Venter hopes to announce completion of the map by late this year, with publication in a scientific journal sometime in the spring. The project is being conducted in close collaboration with researchers at the University of California at Berkeley who have spearheaded fruit-fly gene-mapping for years.

Gerald Rubin, the scientist in charge of the Berkeley project, said the collaboration with Celera has gone well so far, and he hopes it can become a model for greater cooperation in researching human genes. While noting that many difficult steps remain to be accomplished, he said he was optimistic that fruit-fly researchers would have a complete gene map in their hands in a matter of months.

"They're eager for it, and they wish they had it yesterday," he said. "They don't care who does it as long as it's available to them."

Getting to Know the Fruit Fly

About Drosophila melanogaster:

Length: Three millimeters

Number of chromosomes: Four pairs

Number of genes: About 12,000 (human genome has about 70,000 genes)

About the sequencing:

* Heading Celera Genomics's project: J. Craig Ventner

* How long the physical sequencing took: About three months

SOURCES: Celera Genomics, the Exploratorium