To investigate the ability of prokaryotic microorganisms to activate strategies in adapting themselves to the environmental stress induced by exposure to extremely low frequency electromagnetic fields (ELF‐EMF), cultures of Escherichia coli ATCC 700926 exposed at 50 Hz EMF (0.1, 0.5, 1.0 mT), and the respective sham‐exposed controls were studied for: the total and culturable counts, the viability status, the antimicrobial susceptibility pattern, the morphological analysis, the genotypical and transcriptional profile. Exposed samples and controls displayed similar total and culturable counts, whereas an increased cell viability was observed in exposed samples re‐incubated for 24 h outside of the solenoid compared to the corresponding controls. An exposure to 50 Hz EMF of 20–120 min produced a significant change of E. coli morphotype with a presence of coccoid cells also aggregated in clusters after re‐incubation of 24 h outside of the solenoid. Atypical lengthened bacterial forms were also observed suggesting a probable alteration during cell division. No changes among DNA fingerprintings and some differences in RNA‐AFLP analysis were observed for each 50 Hz EMF intensities evaluated. Our results indicate that an exposure to 50 Hz EMF acts as a stressing factor on bacteria which can represent a suitable model to investigate acute and chronic effects related to ELF‐EMF exposure. Bioelectromagnetics 29:302–311, 2008. © 2008 Wiley‐Liss, Inc.