Utilizing a carbon filament doping source, we prepared two-dimensional hole gases in a symmetric quantum-well structure in the GaAs/AlGaAs heterosystem. Low-temperature hole mobilities up to 1.2x10(6) cm(2)/V s at a density of 2.3x10(11) cm(-2) were achieved on GaAs (001) substrates. In contrast to electron systems, the hole mobility sensitively depends on variations of the quantum-well width and the spacer thickness. In particular, an increase of the quantum-well width from an optimal value of 15 nm to 18 nm is accompanied by a 35% reduction of the hole mobility. The quality of ultrahigh-mobility electron systems is not affected by the employed carbon-doping source. (c) 2005 American Institute of Physics.