Huntington disease is caused by mutational expansion of the CAG trinucleotide within exon 1 of the huntingtin (Htt) gene. Exon 1 spanning N-terminal fragments (NTFs) of the Htt protein result from aberrant splicing of transcripts of mutant Htt. NTFs typically encompass a polyglutamine tract flanked by an N-terminal 17-residue amphipathic stretch (N17) and a C-terminal 38-residue proline-rich stretch (C38). We present results from in vitro biophysical studies that quantify the driving forces for and mechanisms of polyglutamine aggregation as modulated by N17 and C38. Although N17 is highly soluble by itself, it lowers the saturation concentration of soluble NTFs and increases the driving force, vis-à-vis homopolymeric polyglutamine, for forming insoluble aggregates. Kinetically, N17 accelerates fibril formation and destabilizes nonfibrillar intermediates. C38 is also highly soluble by itself, and it lends its high intrinsic solubility to lower the driving force for forming insoluble aggregates by increasing the saturation concentration of soluble NTFs. In NTFs with both modules, N17 and C38 act synergistically to destabilize nonfibrillar intermediates (N17 effect) and lower the driving force for forming insoluble aggregates (C38 effect). Morphological studies show that N17 and C38 promote the formation of ordered fibrils by NTFs. Homopolymeric polyglutamine forms a mixture of amorphous aggregates and fibrils, and its aggregation mechanisms involve early formation of heterogeneous distributions of nonfibrillar species. We propose that N17 and C38 act as gatekeepers that control the intrinsic heterogeneities of polyglutamine aggregation. This provides a biophysical explanation for the modulation of in vivo NTF toxicities by N17 and C38.