Ultrasonic attenuation and sound velocity have been measured for a single crystal of the Gd_<0.7>Y_<o.2>Lu_<0.1> alloy over the temperature range 160−230 K which includes the ferromagnetic and screw spin antiferromagnetic states. The anomalies observed in attenuation and velocity, which are present only when the specimen is heated up from the ferromagnetic state, are attributed to screw spin antiferromagnetic domains. This view is also confirmed by thermal cycling and magnetic field cycling studies.