Processing speed (PS) is one of the basic elements of cognitive functions and has been regarded as a "common mechanism" which mediates general cognitive decline in aging. The present study of Australian twins (117 monozygotic pairs, 98 dizygotic pairs, and 42 single twins aged 65 years and over), estimated the genetic influences in five measures of PS: Digit Symbol Coding (DS), Trail Making Test A (TMTA), Stroop color naming and word reading (Stroop), Simple Reaction Time (SRT) and Complex Reaction Time (CRT); and their covariation with general cognitive ability (GCA): reasoning, problem-solving, and memory. Additive genetic factors explained 62% of the variance in DS, 42% in TMTA, 57% in Stroop, and 48% and 35% in SRT and CRT, respectively. Quantitative genetic modeling showed that all of the covariation between the five PS measures and GCA could be explained by one common genetic factor, while the covariation between the PS measures was partly explained by non-shared environmental as well as genetic influences. The genetic correlation among the PS measures was strongest for DS and TMTA, and between the PS measures and GCA was strongest for DS. These findings suggest that the different PS measures, as well as GCA were to a large extent influenced by the same set of genes and that the relationship between PS and GCA is entirely due to shared-genetic influences.