Higher eukaryotes sense microbes through perception of pathogen-associated molecular patterns (PAMPs). The flagellin receptor FLS2 represents so far the only known pattern recognition receptor (PRR) in Arabidopsis. Arabidopsis plants detect a variety of PAMPs including specific epitopes of the bacterial proteins flagellin and EF-Tu. Here, we show that flagellin and EF-Tu activate a common set of signalling events and defence responses, but without clear additive or synergistic effects. Treatment with either PAMP results in increased receptor sites for both PAMPs, a finding employed in a reverse-genetic approach to identify the receptor kinase EFR as the EF-Tu receptor Transient expression of EFR in Nicotiana benthamiana results in formation of specific binding sites for EF-Tu, and responsiveness to this PAMP. Arabidopsis efr mutants show a higher frequency of T-DNA transformation by the bacterium Agrobacterium tumefaciens, revealing a role for EF-Tu perception in restricting this plant pathogen. These results demonstrate that EFR is the receptor for EF-Tu and that plant defence responses induced by PAMPs like EF-Tu reduce transformation by Agrobacterium.