Fabrication of highly ordered superstructures of single-walled carbon nanotubes (SWNTs) is of great interest for a wide range of potential applications. Here, we investigate the selective distribution of individually isolated SWNTs (1D nanoparticles with very large aspect ratios) in Pluronic P84/water/p-xylene ternary systems in a lamellar phase by contrast variation small-angle neutron scattering measurements. Hydrophilically functionalized SWNTs (p-SWNTs) with an aspect ratio of ca. 100 are mixed with P84/water/p-xylene systems prepared with two opposite neutron contrasts, a positive contrast for which the neutron scattering length density of the apolar domain is higher than that of the polar domain and a negative contrast for which the relative scattering length density is opposite. The neutron scattering intensity of the first-order Bragg peak, after correcting for the effect of p-SWNT-induced diffuse interface, increases with addition of p-SWNTs in the positive contrast samples and decreases in the negative contrast samples. This shows that p-SWNTs, of which the length is ca. 70 times larger than the thickness of polar domain while its diameter is comparable to the polar domain thickness, are selectively distributed in the polar domains of the P84/water/p-xylene lamellar phase.