Pastorius was most identified by his use of two well-worn Fender Jazz Bass from the early 1960’s: A 1960 Fretted, and a 1962 Fretless. The fretless was originally a fretted bass, from which he removed the frets and used wood filler to fill in the grooves where the frets had been, along with the holes created where chunks of the fretboard had been taken out. Jaco then sanded down the fingerboard, and applied several coats of marine epoxy (Petit’s Poly-poxy) to prevent the rough Rotosound RS-66 roundwound bass strings he used from eating into the bare wood. Even though he played both the fretted and the fretless basses frequently, he preferred the fretless, because he felt frets were a hindrance, once calling them “speed bumps.”
The “Jaco growl” is obtained by using the bridge pickup exclusively and plucking the strings close to it. Additionally, Jaco used the “Variamp” EQ (equalization) controls on his two Acoustic 361 amplifiers (made by the Acoustic Control Corporation of Van Nuys, California) to boost the midrange frequencies, thus accentuating the natural growling tone of his fretless passive Fender Jazz Bass and roundwound string combination. His tone was also colored by the use of a rackmount chorus effect (an offboard sound modification device similar to a phase shifter) which gave a slight doubling effect, and his use of the original Acoustic brand bass amplifier. He would often use the fuzz control built in on the Acoustic 361. Other effects he used live were his octaver (an offboard effect pedal which provides a 2nd tone an octave lower) and his MXR sampler pedal which can be heard on his live solo spot with Weather Report, ‘Slang’ (Jaco loops a short extract of playing, and then solos over it).
Pastorius used natural and artificial/false harmonics to extend the range of the bass (exemplified in the bass solo Portrait of Tracy off of his eponymous album) and could achieve a horn-like tone through his playing technique. Both of his Fender basses were stolen shortly before he entered Bellevue hospital in 1986; they were never recovered. Jaco also had two Jaydee Basses made for him shortly before he died; a fretted and a fretless.
In the early to mid-1980s, Pastorius began to experience mental health problems, including symptoms of manic depression. These were worsened by heavy drug and alcohol use. Although his on-stage and off-stage antics were already well-documented, his mental health and addiction problems exacerbated his unusual and often bizarre behavior and his musical performances also suffered.