The structures of the isostructural La3In5 and beta-Y3In5, Pu3Pd5-type, have been refined and analyzed (Cmcm, Z = 4, a=10.345(4) Angstrom, 9.840(3) Angstrom, b=8.424(6) Angstrom, 8.002(9) Angstrom, c=10.643(6) Angstrom, 10.315(5) Angstrom, R/R(W) = 1.9/2.1, 2.4/2.4%, respectively). A somewhat slow first-order phase transition of the latter to (alpha)Y3In5 (Tm3Ga5-type) occurs in the neighborhood of 900-1000 degrees C (Pnma, Z = 4, a 12.219(7) Angstrom, b=10.328(7) Angstrom, c = 6.424(3) Angstrom, R/R(W) = 5.0/5.1%). The former structures contain well-defined indium square pyramids (C-2v d(In-In) similar to 3.0-3.2 Angstrom) with minimum intercluster separations of 3.43 and 3.58 Angstrom in the larger lanthanum compound, 0.2-0.3 Angstrom less in beta-Y3In5. Clusters in the low-temperature (alpha)Y3In5 phase have been twisted and joined by short bonds at trans-basal positions into chains that are more weakly interconnected (as above) into a three-dimensional structure. All three phases are poorly metallic (rho(295) similar to 90, 47, 47 mu Omega cm, respectively) and Pauli-paramagnetic-like, La3In5 having the smallest chi(M) value (4 x 10(-4) emu mol(-1)). The apparent In-5(9-) cluster in La3In5 can in the classical limit be readily described as a closed shell, nido-deltahedron, and the compound structurally, as a Zintl phase. The obviously important bonding that gives such clear definition of clustering makes it appropriate to refer to La3In5 as a "metallic Zintl phase", downplaying the delocalization of a few of the least tightly bound electrons.