Dr. Jaffe has readily acknowledged gaps in the investigation. Dr. Acer was interviewed only once, agreeing then to give a blood sample for the genetic tests. The next day he hired a lawyer, who did not allow another interview. Dr. Acer closed his office in August 1989 and died from AIDS a year later. His appointment books and many patient records, which would have been immensely helpful in the investigation, were destroyed when he closed his practice.

The disease-control agency based its conclusion in part on comparisons of sequencing of the 7 cases and 32 unrelated ones in southern Florida. The agency's tests were challenged as inconclusive last year in an article in the British scientific journal Nature by a team headed by Dr. Ronald W. DeBry of Florida State University in Tallahassee. The report contended that there was as much statistical likelihood for transmission within the community as in the dental practice.

This May, another report in Nature, which critics have overlooked, greatly strengthened the agency's position. The authors, Dr. David M. Hillis and Dr. John P. Huelsenbeck of the University of Texas in Austin, independently analyzed additional sequences from other infected individuals in southern Florida and the older ones that the centers and Dr. DeBry's team had sequenced and analyzed for earlier reports. The Texas researchers reported that their methods of analysis strongly supported the centers' findings and showed that the conclusions were even stronger than Federal scientists had reported.

The exchange concerned highly technical points and reflected interpretations made from different methods, Dr. DeBry said. He said there was "not any contentious debate" between himself and Dr. Hillis.

When the centers asked Dr. Acer for a blood sample to compare his H.I.V. with Ms. Bergalis's, the epidemiologists assumed the strains would not match, Dr. Jaffe said.

The test that did find a match is the same one that, contrary to widely publicized reports, scientists have used since H.I.V. was first detected a decade ago to help track the spread of AIDS. For example, the technique was used to track the spread of H.I.V. through contaminated syringes to 90 children in two Russian hospitals.

Epidemiologists must rely on laboratory tests in their investigations, and this one was aided by the fact that DNA sequence tests can be applied to H.I.V., because it mutates and rapidly diversifies, said Dr. Gerald Myers, who heads the program at Los Alamos. Sequences from about 2,000 patients have been deposited in the Los Alamos laboratory, and no geographical clusters like that suggested to have existed in southern Florida have been identified. Dr. Myers said his laboratory had collected more than enough samples from cities for its computers to detect such a phenomenon if it existed. He has looked for chance groupings in about 200 samples from Amsterdam, Baltimore and Miami and found none.