Jeroen Paul Thesseling

Jeroen Paul Thesseling (born 13 April 1971) is a Dutch bass player specializing in fretless bass guitar, best known for his work with technical death metal bands PESTILENCE and OBSCURA. He began classical violin lessons at the age of seven, immersing himself in orchestral and ensemble performances during his early years. His classical training continued until he was 16, providing a foundation in playing techniques, counterpoint and melodic articulation. Jeroen transitioned from violin to bass guitar in 1987 and a year later he pursued jazz-oriented bass guitar studies at the ARTEZ School of Music in Enschede, The Netherlands. After playing metal and jazz in his late teenage years he joined the Dutch death metal band PESTILENCE in 1992 recording the band's fourth influential studio album SPHERES (1993), contributing bass lines that blended fusion and metal elements. From 1995 he experimented with microtonal octave divisions on 6-string bass, which resulted in HAFNIUM — a study in 72-tone equal temperament (1999) — and ARGON — a study in 18-tone equal temperament (2000). In 2005 Jeroen recorded his first fretless work with the world-fusion project ENSEMBLE SALAZHAR. Since that same year he has played exclusively fretless bass. He joined the German technical death metal band OBSCURA in 2007, contributing to their second studio album COSMOGENESIS (2009) and third studio album OMNIVIUM (2011), where his fretless playing became a hallmark of the band's progressive sound. After a 15-year hiatus he rejoined PESTILENCE in 2009 and contributed to the band's fifth studio album DOCTRINE (2011). Since 2011 he plays exclusively 7-string fretless basses built by German bass luthier WARWICK, which are strung with the subcontra tuning F#(0); B(0); E(1); A(1); D(2); G(2); C(3). In 2014 he co-founded the jazz-fusion project SALAZH TRIO, featuring legendary Cuban jazz drummer Horacio Hernández, with whom he released the debut work CIRCULATIONS (2017). In 2019 Jeroen co-founded the instrumental progressive metal-fusion quartet QUADVIUM with fellow fretless bassist Steve Di Giorgio (TESTAMENT, DEATH), emphasizing dual-bass interplay in an experimental framework. In 2020 he rejoined OBSCURA, participating in the recording of their sixth studio album A VALEDICTION (2021). In 2024 QUADVIUM finished their much-anticipated recordings and released their debut studio album TETRADŌM (2025) via Agonia Records.