This paper presents the first application of the autocorrelation-width technique (Bencze A and Zoletnik S 2005 Phys. Plasmas 12 052323) to detect and characterize the microturbulence-zonal flow system using fluctuating signals ( , ) measured by arrays of single-tip Langmuir probes in the CASTOR tokamak. Radially localized (≈1 cm) random flow structures have been clearly observed with a lifetime of ≈1 ms and a long-range poloidal correlation, extending over at least one-fourth of the poloidal circumference. © 2006 IOP Publishing Ltd.