Stream segregation or fission of the fast alternating tone sequence ABAB is known to occur if there is a sufficient frequency difference between the tones A and B. In this paper it will be shown that level difference instead of frequency difference can be sufficient to enable the occurrence of fission. The smallest level difference between A and B, ?L?3 dB (2.5-10 tones per sec; tone duration 40 msec). At rates faster than 12 tones per sec a new perceptive phenomenon was observed: the roll effect. It is characterized by the weak tones being heard at double the tempo. The relation with the continuity effect is investigated using alternating sequences with both level and frequency difference between the tones as stimuli.