Poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) was electrospun into ultrafine fibrous nonwoven mats. Different from the conventional electrospinning process, which involves a positively charged conductive needle and a grounded fiber collector (i.e., positive voltage (PV) electrospinning), pseudo-negative voltage (NV) electrospinning, which adopted a setup such that the needle was grounded and the fiber collector was positively charged, was investigated for making ultrafine PHBV fibers. For pseudo-NV electrospinning, the effects of various electrospinning parameters on fiber morphology and diameter were assessed systematically. The average diameters of PHBV fibers electrospun via pseudo-NVs were compared with those of PHBV fibers electrospun via PVs. With either PV electrospinning or pseudo-NV electrospinning, the average diameters of electrospun fibers ranged between 500 nm and 4 mu m, and they could be controlled by varying the electrospinning parameters. The scientific significance and technological implication of fiber formation by PV electrospinning and pseudo-NV electrospinning in the field of tissue engineering were discussed. POLYM. ENG. SCI., 51:1325-1338, 2011. (C) 2011 Society of Plastics Engineers