The present invention relates to a drill bit assembly for fluid-operated percussion drill tools comprising a percussion bit (1) having a head portion (3) formed with an axially extending stub shank (32); axially extending splines (36) on the stub shank (32) slideably engageable with complementary splines (35) formed on a drive chuck (4) whereby rotational drive from the chuck (4) may be transmitted to the stub shank (32); bit retaining means adapted for engagement with complementary retaining means on the stub shank to retain the stub shank in the drill bit assembly; and engagement means on the chuck (4) adapted for connecting the chuck (4) to a drive means (5) of the fluid-operated percussion drill tool. The bit retaining means comprises a bit retaining ring (41), and the complementary retaining means comprises a retaining shoulder (37) on the stub shank, wherein the bit retaining ring is arranged to engage the retaining shoulder to retain the stub shank in the drill bit assembly. A portion (42) of the bit retaining ring (41) adapted to engage the retaining shoulder (37) to retain the stub shank (32) in the drill bit assembly is disposed within the chuck (4).