After conventional incubation (n = 2) or incubation in a 60-Hz modulated 2450-MHz microwave field (n = 2), four Leghorn chickens (Gallus gallus) of both genders were assessed for ability to perceive magnetic fields (dc at 4.0 milliteslas; 60-Hz ac at 1.7 milliteslas rms) by the method of conditional suppression. Each bird operantly responded for rewards of grain while being subjected to a Pavlovian conditioning procedure in which a 90-s presentation of an H field served as a conditional stimulus (CS) that terminated in a brief faradic shock to the feet, which served as the unconditional stimulus (US). Five to seven pairings of CS and US occurred aperiodically during daily, 60-min sessions. A total of six tensession blocks of formal testing was conducted, two blocks with the ac field, two with the dc field, and two with a sham field in which the US alone was presented to provide control baselines of responding. Neither mode of incubation nor gender was a reliable source of variation, but highly reliable changes (Ps < 0.01) in the character of operant responding—increased variability—were observed during presentations of the ac or dc field. The domestic fowl may therefore share with several other avian species a sensitivity to H fields, although artifactual sensory cueing borne of vibration or heating of the H-field source was not definitively ruled out. If real, the birds' sensitivity to the nominal dc field was probably associated with behavioral modulation – with incessant movement in the field by a chicken, especially of its head, during the operant-respondent measures of conditioning. On the basis of our findings and those of other investigators, we believe that detection of H fields by avian species has its locus in the retina or in other neuroanatomical substrates of the visual system.