In the past year there have been many developments in the area of new or improved diagnostic tests for bacterial, parasitic, and viral agents causing gastrointestinal infections. The cause of Whipple's disease has been identified using polymerase chain reaction amplification of bacterial ribosomal RNA, and as a result a new polymerase chain reaction-based diagnostic test has been developed. Rapid polymerase chain reaction-based assays have recently been reported for detection of cholera toxin and Norwalk virus directly in diarrheal stool. Several new tests are now described that provide evidence of infection with Shiga-like cytotoxin-producingEscherichia coliin both hemolytic uremic syndrome patients and those with diarrhea or hemorrhagic colitis. Results of four clinical trials of enzyme-linked immunosorbent assays for rapid detection ofClostridium difficiletoxins A or B or both are evaluated, along with an improved enzyme immunoassay for astrovirus detection. A new chromotrope stain forEnterocytozoon bieneusiand a seven-test key for speciation ofAeromonasare described. This report emphasizes tests with improved sensitivity, specificity, and utility in the clinical microbiology laboratory.