The design of solid substrate fermentors is often limited by large temperature and concentration gradients within the bed. However, because the metabolic rate of white rot fungi on wood chips is much lower than that of most organisms used in other solid substrate fermentations, it is not expected that these effects will significantly limit the design of reactors for biopulping i. e, the microbial degradation of lignin in wood chips. Preliminary design calwlations support this hypothesis. Therefore, it will be possible to operate a biopulping, reactor at a small air velocity and pressure drop, allowing for a great deal of latitude in reactor design. Several possible designs are presented. It is recommended that further research in this area focus on increasing theoretical understanding of the fermentation and on reducing treatment time.