Spiders have captured the interest of scientists for many years because spider silks are among the toughest materials, having properties that surpass some man-made synthetic materials. Spinning recombinant silk to duplicate those properties has proved to be extremely difficult. This is the first known report of spinning recombinant silk fibers in an aqueous environment. The method seeks to keep the protein soluble throughout the process, not unlike the way the spider stores and spins silk. Recombinant silk proteins were produced by bacterial fermentation in which the cell pellets were lyophilized and lysed with organic acid. Silk protein was purified from the lysate by chromatography and processed in dilute denaturing buffer into a fiber spinning solution. Circular dichroism measurements of the silk solutions revealed an increase in beta-sheet content as a function of time. Time-dependent self-association of silk protein was monitored in solution by dynamic light scattering. Furthermore, the observed increase in beta-sheet content and self-association appear to be required for fiber formation. Recombinant silk fibers were 10-60 mum in diameter, water insoluble, and birefringent, indicating molecular orientation within the fiber.