Transcription in vitro of total genomic DNA of Octopus vulgaris resulted in PolIII transcripts of 200 to 500 nucleotides in length. These transcripts were used as probes and, as a result, three different kinds of short interspersed element (SINE) were isolated and characterized. Two SINEs, designated the octopus OR1 and OR2 families, seem to have been derived from tRNA(Arg). The other SINE, designated the octopus OK family, have originated from tRNA, but the parental tRNA species cannot be identified due to sequence divergence from the original tRNA sequence. The OR1 and OR2 families exhibit considerable similarity to one another, in the 5' region of the tRNA-unrelated region as well as in the tRNA(Arg)-related region, an observation that suggests that these two families may have had the same origin in evolution. The three SINEs together constitute at least 6% of the genome of O. vulgaris. Results of a dot hybridization experiment suggest that the OR1 and OR2 families are present in fewer species than the OK family and that the OK family exists in many species of octopodid. The present observations indicate that SINEs have generally been derived from tRNAs in invertebrates, as well as in vertebrates, and that retroposition is widely involved in the genetic and structural variability of invertebrate genomes. It has been demonstrated that tRNA(Arg) and tRNA(Lys) appear to be tRNA species found as progenitors of vertebrate SINEs, and the same preference for progenitor species of tRNAs seems also to be a feature of invertebrate SINEs.