Previous research shows that reinforcement of variable responding will facilitate sequence learning in rats but may interfere with sequence learning in humans. The present study aimed to explore the notion of sequence difficulty in humans by manipulating both sequence length (6-12 items) and task instruction. Participants were randomly allocated to one of two groups: Control and Variable. In the control group sequences were only reinforced if they were the target sequence, in the variability groups sequences were concurrently reinforced on a Variable Interval 60-s schedule if the just entered sequence met a variability criterion and for every production of the target sequence. With the instructions - to earn as many points as possible by figuring out the correct sequence - the six-item sequence (122121) Control group were most likely to produce the target sequence, while for the twelve digit sequence (221112211121) there was no difference between the two groups. The Variable group were most likely to produce the target sequence for the intermediate nine-digit sequence (112212121). Preliminary findings on the same task without instructions suggest comparable findings. The use of sequence length as a definition of sequence difficulty in both the current and previous studies and the potential role of instructions are discussed.