Context :

Aims : We study the spectral and temporal behavior of the High Mass X-ray Binary A 0535+26 during a ‘ pre-outburst flare ’ which took place \sim 5 \mathrm { d } before the peak of a normal ( type I ) outburst in August/September 2005 .
We compare the studied behavior with that observed during the outburst .

Methods : We analyse RXTE observations that monitored A 0535+26 during the outburst .
We complete spectral and timing analyses of the data .
We study the evolution of the pulse period , present energy-dependent pulse profiles both at the initial pre-outburst flare and close to outburst maximum , and measure how the cyclotron resonance-scattering feature ( hereafter CRSF ) evolves .

Results : We present three main results : a constant period P = 103.3960 ( 5 ) \mathrm { s } is measured until periastron passage , followed by a spin-up with a decreasing period derivative of \dot { P } = ( -1.69 \pm { 0.04 } ) \times 10 ^ { -8 } \mathrm { s s ^ { -1 } } at MJD 53618 , and P remains constant again at the end of the main outburst .
The spin-up provides evidence for the existence of an accretion disk during the normal outburst .
We measure a CRSF energy of E _ { \mathrm { cyc } } \sim 50 \mathrm { keV } during the pre-outburst flare , and E _ { \mathrm { cyc } } \sim 46 \mathrm { keV } during the main outburst .
The pulse shape , which varies significantly during both pre-outburst flare and main outburst , evolves strongly with photon energy .

Conclusions :