Defective RNA molecules (D-RNAs) are being studied in several plant RNA virus groups. In the genus Bromovirus, D-RNAs have been described for Broad bean mottle virus (BBMV), they are formed by a single internal deletion in the RNA2 and have been generated de novo by serial passages. In contrast, in Brome mosaic virus (BMV) D-RNAs are generated by a single or double internal deletion in the RNA3 without serial passages in their hosts. In this work the external effects of the host and the growth temperature in the generation and accumulation of D-RNAs in BBMV, BMV and Cowpea chlorotic mottle virus (CCMV) are studied. The BBMV and BMV D-RNAs were generated and accumulated with or without serial passages of the viruses in different hosts and cultivars. Plants grown at 12, 16, 20 and 24 deg C in a growth chamber and in a greenhouse (22 +/- 5 deg C) generated D-RNAs after being inoculated with virusfree D-RNAs, with and without passages. The D-RNAs observed in BBMV and BMV presented some common characteristics: both were formed de novo after serial passages or without passages, their deletion borders had short repeated and palindrome sequences that favour recombination. In addition, growing inoculated plants at lower temperatures greatly facilitated the generation and accumulation of D-RNAs. The CCMV did not generate defective molecules in cowpea (Vigna unguiculata) and Nicotiana benthamiana plants after several serial passages or without passages. This is the first time that D-RNAs have been generated in BBMV without passages and BMV with serial passages.