Induction of tetraploid amago salmon was attempted by treating inseminated eggs with hydrostatic pressure shock (650 kg/cm2, 6 min duration) in 1996 and 1997. Based on the apperance of four nucleoli in the epithelial cells of pectoral fins, putative tetraploids were selected among six-month-old juveniles developed from pressure-shocked eggs of both year classes. At the age of twenty three months, the surviving ten females of the 1996 year class were flow-cytometrically determined to be diploid by measuring DNA content of erythrocytes. However, both diploids and triploids were detected in the progeny, when one of these diploid females was crossed with normal diploid males. Other females gave no triploid progeny. Among eleven individuals examined in the one-year-old 1997 year class salmon, a diploid-tetraploid mosaic male was found, but the others were diploid. All the progeny examined were diploid, when the eggs of a normal diploid were fertilized by spermatozoa of this mosaic male. The present results suggest the possible production of tetraploid and other polyploid lines using diploid gametes of fish which developed from the chromosome manipulation to inhibit first cleavage for induction of tetraploids but were determined to be diploid in adult stage by cellular size and DNA content of somatic cells. Such chromosomally-manipulated fish are likely to be mosaics including tetraploid cells in their germ line, in spite of a large population of diploid cells in soma. The present results also demonstrate a different case in which the apparent diploid-tetraploid mosaic generated only haploid spermatozoa.