The reproductive biology of the least chub Iotichthys phlegethontis (Cope) was studied from June 1976 to March 1978. Fish from both field and laboratory populations of the Leland Harris Spring Complex, Juab Co., Utah were utilized in the study. Reproduction in 1977 occurred from April to July in the field population and from April through August in the laboratory population. Reproduction was determined from gross examination and weight measurements of testes and ovaries, breeding coloration in males and diameter measurements of ova. Males and females matured at about the same size, 28 to 30 mm TL. The number of mature ova produced during spawning ranged from 283 to 2752 for females 31 to 46 mm TL. Mature ova ranged in size from 0.8 to 1.28 mm. Least chub are partial and intermittent spawners. This conclusion is supported by ova diameter measurements and calculations of the duration and peak period of spawning. Least chub are polyandrous broadcast spawners over vegetation. The eggs are demersal and adhesive. The egg incubation period was 2.1 days at 22.2 C. The eleutheroembryo are approximately 4 mm in length. They are not photophobic and appear to have cement glands. Specific characteristics of reproduction are discussed in relation to ecological conditions of the habitat.