An in-line viscometer was developed to determine the rheological properties of biogas slurries at a full-scale biogas plant. This type of viscometer allows the investigation of flow characteristics without additional pretreatment and has many advantageous aspects in contrast to the rotational viscometer. Various effects were studied: alterations in the feedstock structure, increasing total solid (TS) of the slurry and the disintegration of the feedstock on the rheological properties. The results indicate that the Power-Law model is sufficient for the description of the flow curve of biogas slurries. Furthermore, the use of more fibrous materials increases in viscosity. The increase in TS of 10.1-15.1% resulted in a sharp increase of the viscosity. The mechanical disintegration of the feedstock positively influenced the rheological properties, but the effects were more apparent at higher TS. (C) 2014 Elsevier Ltd. All rights reserved.