This study will assess the efficacy of Botulinum Toxin B (Myobloc®) injected into the lumbar paravertebral muscles for reducing pain and disability in subjects suffering from acute low back pain (duration 3 to 6 weeks) arising from an identifiable muscle strain injury or back trauma. The treatment modality and techniques used are based upon three successful prior open-labeled pilot studies done by this research group investigating the effect of Botulinum Toxin A (BOTOX®) on relief of chronic low back pain. This study, however, will employ a prospective, double-blind, randomized, placebo controlled trial to control for any placebo or mechanical trigger-point injection effects. Subjects will also be assessed for 3 months to define the duration of efficacy of Myobloc®. Sixty subjects will be randomly assigned to one of two groups and will then receive either Myobloc® or placebo injection into the lumbar paravertebral muscles. The subjects will be assessed using validated scales for pain and disability prior to injection and weekly to monthly thereafter for three months. All subjects will continue to receive any medication or physiotherapy per standard of care but those treatments will be recorded and controlled for in the data analysis.