Jeroen Thesseling, born 13 April 1971, followed his first classical violin lessons at the age of six and started his study bass guitar in 1988 at the Conservatory in Enschede, the Netherlands. After playing jazz in his late teenage years he collaborated with the Dutch group PESTILENCE from 1992 till 1994, recording their jazz/ fusion-influenced album 'SPHERES' (1993). Always willing to experiment and to deepen his knowledge of other genres, he began studying microtonal music in 1995 that resulted in recording 'HAFNIUM' - study in 72-tone equal temperament (1999) and 'ARGON' - study in 18-tone equal temperament (2000). The period following, contemporary classical music inspired him to focus mainly on fretless bass. In 2005 he recorded with the studio ensemble SALAZH. From 2007 till 2011 he collaborated with the German group OBSCURA, recording the albums 'COSMOGENESIS' (2009) and 'OMNIVIUM' (2011). During the period 2009-2012 he rejoined PESTILENCE after a 15-year break, recording the album 'DOCTRINE' (2011). Since 2011 German bass manufacturer WARWICK built fretless 7-string instruments for him that are strung with a low F#-tuning. Jeroen is a member of the board of the American Festival of Microtonal Music and is involved with several projects, presenting the instrument in an unconventional role within contemporary music.