This paper describes the development of an instrument based on implicit criteria currently used by mental nurses in deciding whether a nurse is a good or a poor mental nurse. Existing implicit means of assessing mental nurses in the form of ward reports have been found to be inadequate. The study consisted of two stages: firstly, the production of a preliminary questionnaire using statements describing good and poor mental nurses obtained by interviewing mental nurses; secondly, the production of an assessment instrument, the Mental Illness Nurses' Evaluation Test, derived from the results of factor analysing responses of mental nurses to the preliminary questionnaire. The criteria used to evaluate mental nurses that emerged in order of importance were benevolence‐malevolence to other nurses, amiable‐irascible, regard‐disregard for patients, confident‐uncertain, directive‐non‐directive and professional‐unprofessional. The study exposes current values used in judging mental nurses and constitutes a first important step in developing a valid and reliable instrument in assessing mental nurses in terms of their pr