A review of the literature has shown that there was no reliable method of measuring the flow properties of fluids in the range of viscosity 106to 1012poises where the flow is ``generally viscous.'' The theory of the penetrometer has been developed in this paper to a point where absolute viscosities may be determined both for viscous flow (viscosity independent of rate of shear) and generally viscous flow (viscosity dependent on rate of shear). The new penetrometer method was checked (1) with the capillary viscometer for viscous flow using coal tar pitch, and (2) with the revised axially moving cylinder method for generally viscous flow using a high melting blown asphalt. For any one temperature, the flow properties of a generally viscous material are defined by the straight line relationship between log shearing stress and log rate of shear. From this relationship viscosity may be calculated at any desired rate of shear. The slopenof this line is required in the calculation of rate of shear both for the penetrometer method and for any other type of standard viscometer method. An independent means for observingnwas developed for the penetrometer and for the moving cylinder method. It is shown that earlier formulas for standard instruments based on viscous flow are invalid when applied to fluids showing generally viscous flow properties. The power function law relating shearing stress and rate of shear has been found to hold for the asphalt studied over the range of rate of shear from 10−5to 10 reciprocal seconds. In addition to speed of operation and precision, the penetrometer has the advantages of minimizing the elastic effects and reproducibility without extensive heat treatment and preworking. Entire shearing stress‐rate of shear curves can be determined with one load and one penetration using a succession of time intervals.