Biostomp is a new musical interface that relies on the use mechanomyography (MMG) as a biocontrol mechanism in live performance situations. Designed in the form of a stomp box, Biostomp translates a performer's muscle movements into control signals. A custom MMG sensor captures the acoustic output of muscle tissue oscillations resulting from contractions. An analog circuit amplifies and filters these signals, and a micro-controller translates the processed signals into pulses. These pulses are used to activate a stepper motor mechanism, which is designed to be mounted on parameter knobs on effects pedals. The primary goal in designing Biostomp is to offer a robust, inexpensive, and easy-to-operate platform for integrating biological signals into both traditional and contemporary music performance practices without requiring an intermediary computer software. In this paper, we discuss the design, implementation and evaluation of Biostomp. Following an overview of related work on the use of biological signals in artistic projects, we offer a discussion of our approach to conceptualizing and fabricating a biocontrol mechanism as a new musical interface. We then discuss the results of an evaluation study conducted with 21 professional musicians. A video abstract for Biostomp can be viewed at vimeo.com/biostomp/video.