Qualitative relationships between agitation levels and medium viscosity, Aspergillus niger morphology and phytase production were investigated in submerged and solid-state fermentations. Overall phytase production increased with increasing shaker speeds from 150 to 300 rpm, although specific growth rates and phytase production rates were higher at 150 rpm for 72 h from inoculation. Fungal morphology was greatly influenced by agitation with the morphological forms of small pellets and entangled mycelia predominating at 150 rpm, while the free filamentous form was obtained at 300 rpm. Upon inoculation of SSF, increased productivities were obtained from inocula grown at 150 rpm. A shift towards the filamentous growth form was observed when guar gum was added to the liquid media, which increased the viscosity from 2 000 cp (medium without gum addition) to 52 000 cp (addition of 1 g/L gum). At both shaker speeds, with the effect being more pronounced at 300 rpm, phytase production increased with gum addition. Specific growth rates for the first 72 hours from inoculation, were higher at 150 rpm, irrespective of gum concentration, while solid-state fermentations inoculated with these cultures led to higher amounts of phytase compared to those obtained from 300 rpm inocula.