Outbreaks of infectious diseases have resulted in high mortality and huge economic losses in penaeid shrimp culture. Interest in understanding shrimp immunity has increased because of its importance in disease control. Here we report cDNA cloning of a Toll receptor from the white shrimp Litopenaeus vannamei. L. vannamei Toll (lToll) is 926 residues, with a putative signal peptide of 19 residues. The protein contains distinct structural/functional motifs of the Toll like receptor (TLR) family, including an extracellular domain containing 16 leucine-rich repeats (LRRs) flanked by cysteine-rich motifs and a cytoplasmic Toll/interleukin-1 receptor (TIR) domain. The lToll TIR domain showed high similarity to Apis mellifer Toll and Drosophila melanogaster Toll, with 59.9% and 54.3% identity, respectively. Reverse-transcription polymerase chain reaction (RT-PCR) analysis showed that lToll was expressed in hemocyte, gill, heart, brain, stomach, intestine, pyloric caecum, muscle, nerve and spermary, with a lower expression level in eyestalk and hepatopancreas. Identification of lToll will help to elucidate the Toll pathway in shrimp innate immunity.