Joint work with John O. Kessler (Physics Department, University of Arizona, Tucson, AZ 85721). Keywords: bacteria, interacting self propelled particles, collective behavior, PIV, bio-fluid-dynamics Abstract: Spatial order and fast collective coherent dynamics of populations of swimming bacteria emerges from local interactions and from flows generated by the organisms’ locomotion. The transition from dilute, to intermediate, to high concentrations of cells will be demonstrated by movie clips. Analyses of these data, presented as probability density functions for swimming velocity, show that the low concentration phase which exhibits swimming speeds characteristic of individual bacteria, arrives at the anomalously high speed phase, ZBN, the ZoomingBioNematic, via an intermediate phase that exhibits surprisingly low mean speeds. The origin of this phenomenon relates to scattering and the known dynamics of velocity flipping. Particle Imaging Velocimetry (PIV) was used for analysis of mixing, and of collective velocities, correlated with alignment within coherent patches. Supported by the Department of Energy, grant DOE-W-31-109-ENG-38.