Ilyodon whitei, a small fish endemic to the Balsas River Basin, and even though it has been declared as Least Concern by the International Union for Conservation of Nature populations are severly fragmented and decreasing in many places. Despite the above, very little is known about the biological characteristics of this species. The aim of this study was to describe the population and reproductive structure of Ilyodon whitei in the Amacuzac River, Morelos, Mexico. To do this, we analyzed 1493 specimens from the Ichthyological Collection of the Centro de Investigaciones Biológicas, Universidad Estatal de Morelos, corresponding to five collection sites over one annual cycle. We evaluated size structure, sex ratio, length-weight relationship, gonadal maturity stage, size at first maturity, fertility, morphological indexes related to reproduction, and reproductive habitat. A spatial and seasonal pattern was observed in the size-class frequency distribution. The overall sex ratio was 1.2:1 (Females:Males). The results of the analysis of the length-weight relationship in both sexes by sites and seasons showed different types of growths. There was a spatial and seasonal pattern in the stages of gonadal development in both sexes. The length at maturity was 51.54 mm (females) and 48.12 mm (males). Fertility ranged between 9 and 42 embryos per female, averaging 18. Anthropogenic activities that have modified the Amacuzac River have caused the current population sizes of Ilyodon whitei to be very low. Since, as reported in the literature, goodeid species have a low tolerance to pollution and cannot reproduce successfully in disturbed waters, notwithstanding the foregoing, in Morelos there are no underlying specific conservation actions to maintain this species and the biodiversity of the Amacuzac River. There are no data available on aspects of reproduction of this species, so this study provides information for the first time, therefore, important information is provided that will allow carrying out strategies for the management and conservation of Ilyodon whitei.