A 4-m high, 15.24-cm diameter semi-batch bubble column connected to one of three perforated plate gas distributors with open area ratios A = 0.57%, 0.99%, and 2.14% is employed to study gas holdup and flow regime in Rayon and Nylon fiber suspensions. This study determines the effect of superficial gas velocity, fiber type, fiber mass fraction, fiber length, and aeration plate open area ratio on gas holdup and flow regime transition in various fiber suspensions.;Experimental results show that gas holdup increases with increasing superficial gas velocity, and the plot of gas holdup vs. superficial gas velocity depends on aeration plate open area. Gas holdup decreases with increasing fiber mass fraction and fiber length.;It is found that Nylon fiber is not an appropriate model system because gas holdup has been shown to vary with time. This phenomenon is mainly attributed to the fact that proprietary additives on the Nylon fiber surface modify the fiber suspension rheology and liquid surface tension with time.;Additional results show that gas holdup increases with aeration plate open area ratio when A ≤ 1%; when aeration open ratio was further increased (e.g., A = 2.14%), gas holdup decreases. Contributions to the decrease in gas holdup with increasing open area ratio are discussed.;Homogeneous, transitional, and heterogeneous flows are generated using three different aeration plates in air-water and low fiber mass fraction suspensions, and the superficial gas velocity at which the flow regime transition occurs depends on fiber length and aeration plate open area. The flow pattern changes to pure heterogeneous flow when the fiber mass fraction is high. The critical fiber mass fraction at which pure heterogeneous flow is observed depends on fiber length and aeration plate open area.;Finally, three gas holdup models are developed corresponding to different flow regimes by using data in this study (D = 15.2 cm). The models can reproduce most of the data within +/-15%. These models are also successfully extended to reproduce gas holdup within +/-15% in a similar bubble column with D = 32.1 cm.