This paper investigates the effects of fiber mass fraction and length on gas holdup and flow regime transition. The experiments are performed using three different Rayon fiber lengths (3, 6, and 12 mm) over a range of superficial gas velocities (U-g less than or equal to 18 cm/s) and a range of fiber mass fractions (0 less than or equal to C less than or equal to 1.8%) in a 15.24 cm diameter bubble column, in which air is continuously bubbled through a fixed volume of a fiber suspension. Experimental results show that increasing the fiber mass fraction tends to decrease the overall gas holdup when the fiber mass fraction is less than a certain value, which depends on fiber length. Fiber length influences the gas holdup when C less than or equal to 1.4%, and increasing fiber length reduces the overall gas holdup. Homogeneous, transitional, and heterogeneous flow conditions are observed at low fiber mass fractions, but when C less than or equal to 0.6%, purely heterogeneous flow conditions are observed. The flow regime transition is not significantly affected by fiber length.