1. The fusion of two reproductive cells, particularly when these are both the same size (see isogamy).

2. A form of sexual reproduction seen in some algae (e.g. Spirogyra), some bacteria (e.g. Escherichia coli), and ciliate protozoans. Two individuals are united by a tube formed by outgrowths from one or both of the cells. Genetic material from one cell (designated the male) then passes through the tube to unite with that in the other (female) cell. In bacteria conjugation is initiated and directed by sex factors.