Strain PPAV, a filamentous but nonhelical mollicute, was isolated from aborted apple seeds in France in late 1979. This organism grew well in SP-4 broth, fermented glucose, and required sterol for growth, and most of its properties suggested that it belonged to the genus Mycoplasma. However, it was serologically distinct; in addition, unlike other Mycoplasma species, genome measurements consistently yielded values of about 1,000 MDa (ca. 1,500 kbp), and the organism had a growth temperature optimum of 43°C. A comparison of strain PPAV 16S rRNA sequences with those of other mollicutes revealed a high degree of sequence similarity to a strain of Mycoplasma iowae, which is commonly encountered in poultry. This relationship was confirmed by performing a restriction endonuclease pattern analysis and DNA-DNA hybridization tests. The genome size of type strain 695 of M. iowae was determined to be about 1,000 MDa (1,500 kbp) by renaturation kinetics, a value which is much higher than any other value known in the genus. Additional measurements by pulsed-field gel electrophoresis yielded values of 1,300 kbp for both strain PPAV and M. iowae. Subsequent phenotypic comparisons supported this relationship. Serologic tests with strain PPAV and other strains of M. iowae confirmed the findings of other investigators that this species is serologically heterogeneous. The high optimum temperature for growth of strain PPAV was also shared by a number of M. iowae isolates. Genome size is an inappropriate character for taxonomic assignment to the family Mycoplasmataceae because strain PPAV and other established species in this family are now known to have genomes ranging in size from 1,000 to 1,400 kbp.