A multiplex reverse-transcriptase-PCR (RT-PCR) procedure was developed for the simultaneous detection of porcine epidemic diarrhoea virus (PEDV) and transmissible gastroenteritis virus (TGEV) in preweaning pigs with diarrhoea. The membrane gene of PEDV and the nucleocapsid gene of TGEV were chosen as targets. The PCR products of PEDV and TGEV had molecular sizes of 412 and 612 base pairs, respectively. Primers from PEDV did not react with any TGEV tested and vice versa. In addition, the primers did not react with other pig viruses. The multiplex RT-PCR was able to detect 10 tissue culture-infective doses 50 per cent (TCID50)/ml of PEDV or TGEV with each of the primer sets for PEDV and TGEV, respectively. The RNAS of PEDV and TGEV were detected directly in intestinal and faecal samples from pigs infected experimentally with either virus. The results of the assay correlated well with the results of virus isolation. None of the five control specimens was positive. PEDV was detected in 10 intestinal and nine faecal samples, and among the nine positive faecal samples two were culture-negative. TGEV was also detected in 10 intestinal and nine faecal samples, and among the nine positive faecal samples, three were culture-negative.