New X-ray and optical studies show that thomsonite has a < b < c and its optical orientation is α = b, β = c, and γ = a. The refined cell dimensions of Old Kilpatrick thomsonite are a = 13.051, b = 13.092, and c = 13.263 Å. The systematic absences h+l odd for h0l, h + k odd for odd for hk0, and l odd for 0kl uniquely determine the space group as Pcnn for which the arrangement of the alumino-silicate framework is suggested. The exceptions to the above absences are the 704 reflection (due to Renninger effect) and many weak hk0 reflections which suggest that the space group might be Pcnm or Pcn2. The observed habit, cleavage, and parallel growth are discussed in the light of the proposed framework structure.