Mesostoma erhenbergii are hermaphroditic aquatic flatworms that reside in rivers and lakes across North America and elsewhere. Their primary spermatocytes have a very unusual meiosis (reviewed by Ferraro-Gideon et al. 2014). The cells have four univalent chromosomes, of two different kinds, that stay at the spindle poles, plus three bivalents. Some evidence suggests that the chromosomes are distributed via non-random segregation. For example, univalents move from pole to pole to obtain proper segregation, but they do this more often than necessary to obtain one of each kind at each pole, and often homologous chromosomes switch places with one another (Oakley 1984). Another example is that oriented bivalents sometimes detach and reorient to the opposite poles (Ferraro-Gideon et al 2014). Here we provide further evidence of non-random segregation in Mesostoma spermatocytes. Methods: Testes extracted from living Mesostoma ehrenbergii using a pulled glass needle are expelled onto a coverslip into Ringers solution that contained fibrinogen, and are held in a fibrin clot. Nocodazole (NOC) was perfused during prometaphase. We recorded live cells and studied the time-lapsed images. Results: NOC depolymerizes spindle microtubules. According to standard theories of mitosis, if microtubules are quickly depolymerized the chromosomes should arrest at the metaphase plate and not move. In Mesostoma spermatocytes, however, immediately following drug addition the chromosomes stretch out and, after a few minutes, in 52/59 cells the bivalent kinetochores detach from one pole and all move rapidly toward the other pole (Fegaras and Forer 2018 a and b). We have eliminated some other possibilities, and suggest that the non-random movement to one pole is indicative of normally occurring non-random segregation of chromosomes. Conclusions: There are two suggestive pieces of evidence that there is non-random segregation in Mesostoma spermatocytes. Our experiments add a third piece of evidence. We hope to test further for non-random segregation using micromanipulation techniques to move chromosomes around in the cell, and to mate our strain with European strains of the same species that have different chromosome configurations. References: Fegaras E, Forer A. (2018a). Protoplasma 255: 1205-1224. Fegaras E, Forer A. (2018b). Protoplasma 255: 1401-1411. Ferraro-Gideon J, Hoang C, Forer A. (2014). Protoplasma 251: 127-143. Oakley, H A. (1984). Chromosoma 91: 95-100.