SummaryA polymerase chain reaction (PCR) protocol, previously designed for amplification of a DNA fragment from aster yellows mycoplasmalike organism (MLO), was employed to investigate the detection of MLO DNA in field‐collected andin vitromicropropagated plants. PCR with template DNA extracted from symptomatic, naturally‐infected samples ofBrassica, ChrysanthemumandHydrangea, each yielded a DNA band corresponding to 1.0 Kbp. However, no DNA product was observed when either infectedRanunculus(with phyllody disease) orGladioluswith (symptoms of ‘germs fins’) was used as source of template nucleic acid for PCR; further experiments indicated absence of target DNA in the case ofRanunculusand the presence of substances inGladioluswhich inhibited the PCR. The MLO‐specific DNA was detected by PCR using less than 95 pg of total nucleic acid (equivalent to total nucleic acid from 1.9, ug tissue) in the case of field‐collectedHydrangeaand less than 11.4 pg of nucleic acid (equivalent to total nucleic acid from 19 ng of tissue) in the case of field‐collectedBrassica.The findings illustrate highly sensitive detection of MLOs in both field‐grown andin vitromicropropagated