A minichromosome has originated from the transposing element TE1. This autonomously replicating chromosome contains the structural genes white and roughest, from the Drosophila X chromosome. It arose within a stock carrying TE1 at 45F on chromosome 2. In addition to the w and rst genes, the minichromosome may carry section 45C-45F from chromosome 2. It is inherited by 33%-47% of the offspring. By this criterion it carries a centromere, although the origin of the centromere is unknown. From this minichromosome a still smaller one has originated, probably through the loss of all material from chromosome 2 together with some heterochromatin. At the same time a duplication of white and roughest could have taken place. This chromosome has a strange morphology and is more frequently lost in meiosis than the larger one, but is still transmitted to about 29%-37% of the progeny of one parent heterozygous for the minichromosome. In both cases the flies have variegated eyes, probably as a result of position-effect variegation. The variegation pattern is influenced by factors in the X chromosome. The size of the smaller minichromosome is little more than 1 Mb as determined by pulsed field gel electrophoresis.