The construction and operation of a flux‐biased 80‐Mev betatron is described. This betatron was built as a model to test design features later to be incorporated in a 300‐Mev betatron. Particular attention is paid to factors influencing design and to considerations of magnetic problems which are common to betatrons and synchrotrons such as the construction of field magnets which are uniform, the testing of fields at injection time, the mechanism of electron capture, and the influence of field inhomogeneities. The dimensions and operating characteristics are included together with many comparisons with the 300‐Mev betatron.