In the hypotrichous ciliate Oxytricha nova the cloned precursor gene from the micronuclear genome that encodes actin I is composed of highly disordered blocks of deoxynucleotide sequences. We present and illustrate in detail a recombination model that explains how the actin I gene may be unscrambled during macronuclear development after cell mating. The model was described in a previous publication (Greslin et al.: Proc Natl Acad Sci USA 86:6264-6268, 1989). Here we show the data, described in the earlier publication, that support the model. The data show that scrambling is not an artifact of cloning. They rule against the presence of an unscrambled copy of the actin I gene in the micronucleus, which means that unscrambling must be a part of macronuclear development. Finally, the data prove that the actin I gene in O. trifallax is scrambled in a pattern that resembles the pattern in O. nova.