Gram-positive bacteria, including important human pathogens, produce a variety of cell wall attached proteins that are key to the interactions of the bacterium with its environment. These include proteins potentially involved in infection through host colonisation and biofilm formation. Proteins SasG and Rib, found on the surface staphylococci and streptococci, respectively contain DNA sequence repeats and this has been proposed to enable repeat number variation. We have solved novel structures of the protein repeats, alone and in tandem1,2, demonstrating they form an elongated rod. Repeat number variation would thus lead to differential projection of the N-terminal host colonisation domain from the bacterial surface. Here we propose that Rib and SasG are members of a larger class that we have called Periscope Proteins that are playing a role in regulating the interaction of bacteria with their environment. Multi-domain proteins have been shown previously to usually have adjacent domain sequence identity of <~40% and this was proposed as an evolutionary mechanism to minimise misfolding and aggregation3-5. Thus, Periscope Proteins, which have highly similar (sometimes identical) tandemly-arrayed domains, are highly unusual.