Abstract.We answer an open question related to bisimilarity checking on labelled transition systems generated by prefix rewrite rules on words. Stirling (1996Stirling ( , 1998 proved the decidability of bisimilarity for normed pushdown processes. This result was substantially extended by Sénizergues (1998Sénizergues ( , 2005 who showed the decidability for regular (or equational) graphs of finite out-degree (which include unnormed pushdown processes). The question of decidability of bisimilarity for a more general class of so called Type -1 systems (generated by prefix rewrite rules of the form R a −→ w where R is a regular language) was left open; this was repeatedly indicated by both Stirling and Sénizergues. Here we answer the question negatively, i.e., we show undecidability of bisimilarity on Type -1 systems, even in the normed case. We complete the picture by considering classes of systems that use rewrite rules of the form w a −→ R and R1 a −→ R2 and show when they yield low undecidability (Π 0 1 -completeness) and when high undecidability (Σ 1 1 -completeness), all with and without the assumption of normedness.