In an exploratory study, 36 South African physical science teachers' understanding of basic concepts concerning electric and magnetic fields was studied from a perspective of possible concept confusion. Concept confusion is said to occur when features of one concept are incorrectly attributed to a different concept, in the case of this study to magnetic and electric fields. An example of concept confusion is the perception that a magnetic north pole has an excess of positive charges and consequently attracts negative charges placed in the field. The researchers constructed a framework of 20 interrelated critical aspects of which the scientific topic is composed conceptually. Next, the understanding of concepts and interactions in electric and magnetic fields by 36 teachers of physical science (a subject combining physics and chemistry for grades 10-12), who were enrolled for an in-service subject knowledge upgrading course, was probed through questionnaires and interviews. This approach allowed us to answer the central research question of this study: what alternative understandings do teachers have of the topic of electric and magnetic fields in terms of potential concept confusion? The teachers' understanding does appear to be interpretable in terms of whether or not they distinguish between the critical aspects identified in this study. The results show six categories of aspects of electric and magnetic fields causing teachers' inability to distinguish between the two fields, with a consequent confusion of concepts. These categories are: sources of currents; sources of electric fields; sources of magnetic fields; the effects of electric and magnetic fields on materials; electric and magnetic forces; and the direction of the electric and magnetic forces. Results from this research study may contribute to the enhancement of physical science teacher training and consequently school teaching.