The smaller of the two nuclei found in some Protozoans that is concerned with cell division.

In the protistan group known as the ciliates, the small nucleus containing a single copy of the genome that is used for sexual reproduction.

A micronucleus is the smaller nucleus in ciliate protozoans. In fission it divides by mitosis, and in conjugation furnishes the pairing of gamete nuclei, by whose reciprocal fusion a zygote nucleus is formed, which gives rise to the macronuclei and micronuclei of the individuals of the next cycle of fission.