Do LMXBs contain strange stars?

It has been suggested that low mass X-ray binaries (LMXBs) contain strange (quark) stars. We show that the properties of bulk strange matter are sever ely constrained if the recently discovered kilohertz quasi periodic oscilla tions (QPOs) of the X-ray flux from LMXBs occur at orbital frequencies arou nd strange stars. In particular, the simplest equation of state of strange matter is ruled out if the observed saturation (at 1.07 kHz) of the QPO fre quency in 4U 1820-30 occurs at the orbital frequency either just outside th e surface of the strange star or in the innermost (marginally) stable orbit allowed by general relativity.